diff --git a/CHANGELOG b/CHANGELOG
index 5e7b28009..c4798b911 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,7 @@
 # 3.30.0 (2026-XX-XX)
 
+ * Fix `IntlExtension` letting the pattern derived from a date formatter prototype override an explicit locale
+ * Fix `IntlExtension` not honoring the locale of a date formatter prototype configured with no date and time styles
  * Report a clear error when `random`, `reverse`, `shuffle`, and `split` receive a string that is not valid UTF-8
  * Fix the deprecation about omitting parentheses when calling a macro being reported twice for the same call
  * Add the macro name to the deprecation about omitting parentheses when calling a macro
diff --git a/extra/intl-extra/IntlExtension.php b/extra/intl-extra/IntlExtension.php
index 901e18ea2..4bb3b343f 100644
--- a/extra/intl-extra/IntlExtension.php
+++ b/extra/intl-extra/IntlExtension.php
@@ -186,6 +186,7 @@ final class IntlExtension extends AbstractExtension
     private $listFormatters = [];
     private $dateFormatterPrototype;
     private $numberFormatterPrototype;
+    private array $prototypeDerivedPatterns = [];
 
     public function __construct(?\IntlDateFormatter $dateFormatterPrototype = null, ?\NumberFormatter $numberFormatterPrototype = null)
     {
@@ -473,7 +474,7 @@ final class IntlExtension extends AbstractExtension
 
         if (null === $locale) {
             if ($this->dateFormatterPrototype) {
-                $locale = $this->dateFormatterPrototype->getLocale();
+                $locale = $this->prototypeLocale();
             }
             $locale = $locale ?: \Locale::getDefault();
         }
@@ -500,7 +501,7 @@ final class IntlExtension extends AbstractExtension
             // fall back to the prototype's pattern only when nothing else was given, else it would override the explicit date/time formats;
             // a pattern describes a full datetime rendering, so it cannot be honored by format_date/format_time, which pass 'none' for the other part
             if ('' === $pattern && null === $dateFormat && null === $timeFormat) {
-                $pattern = $this->dateFormatterPrototype->getPattern();
+                $pattern = $this->prototypePattern();
             }
         }
 
@@ -527,6 +528,50 @@ final class IntlExtension extends AbstractExtension
         return $this->dateFormatters[$hash];
     }
 
+    /**
+     * ICU reports "root" for a prototype with no date and no time style, whatever its pattern, and rejects
+     * it as an input locale, so the configured locale is recovered from the calendar ICU opened for it.
+     */
+    private function prototypeLocale(): ?string
+    {
+        $locale = $this->dateFormatterPrototype->getLocale();
+        if (\is_string($locale) && '' !== $locale && 'root' !== $locale) {
+            return $locale;
+        }
+
+        $calendar = $this->dateFormatterPrototype->getCalendarObject();
+        $locale = $calendar ? $calendar->getLocale(\Locale::VALID_LOCALE) : false;
+
+        return \is_string($locale) && '' !== $locale && 'root' !== $locale ? $locale : null;
+    }
+
+    /**
+     * ICU derives a pattern from the locale and the date/time types when none was configured;
+     * a pattern configured to the very value ICU derives is indistinguishable from a derived one.
+     */
+    private function prototypePattern(): string
+    {
+        $pattern = $this->dateFormatterPrototype->getPattern();
+        $dateType = $this->dateFormatterPrototype->getDateType();
+        $timeType = $this->dateFormatterPrototype->getTimeType();
+
+        if (!\is_string($pattern) || '' === $pattern || false === $dateType || false === $timeType) {
+            return \is_string($pattern) ? $pattern : '';
+        }
+
+        $locale = $this->prototypeLocale() ?: \Locale::getDefault();
+        $calendar = $this->dateFormatterPrototype->getCalendarObject();
+        // ICU derives the pattern from the calendar keyword it adds to the locale, which it does not do for a calendar object
+        if ($calendar && \is_int($this->dateFormatterPrototype->getCalendar())) {
+            $locale .= '@calendar='.$calendar->getType();
+        }
+
+        $key = $locale.'|'.$dateType.'|'.$timeType;
+        $this->prototypeDerivedPatterns[$key] ??= (new \IntlDateFormatter($locale, $dateType, $timeType))->getPattern();
+
+        return $pattern === $this->prototypeDerivedPatterns[$key] ? '' : $pattern;
+    }
+
     private function createNumberFormatter(?string $locale, string $style, array $attrs = []): \NumberFormatter
     {
         if (!isset(self::NUMBER_STYLES[$style])) {
diff --git a/extra/intl-extra/Tests/IntlExtensionTest.php b/extra/intl-extra/Tests/IntlExtensionTest.php
index 0fef4b817..9d24fe9b0 100644
--- a/extra/intl-extra/Tests/IntlExtensionTest.php
+++ b/extra/intl-extra/Tests/IntlExtensionTest.php
@@ -194,6 +194,47 @@ class IntlExtensionTest extends TestCase
         $this->assertSame('22:22:00', $ext->formatTime($env, $date));
     }
 
+    public function testFormatterProtoPatternDoesNotOverrideAnExplicitLocale(): void
+    {
+        $env = new Environment(new ArrayLoader());
+        $date = new \DateTime('2019-08-07T23:39:12+00:00');
+
+        // this prototype has no configured pattern, but ICU still reports one
+        $derivedProto = new \IntlDateFormatter('fr_FR', \IntlDateFormatter::LONG, \IntlDateFormatter::SHORT, 'UTC', \IntlDateFormatter::GREGORIAN);
+        $expected = (new \IntlDateFormatter('en_US', \IntlDateFormatter::LONG, \IntlDateFormatter::SHORT, 'UTC', \IntlDateFormatter::GREGORIAN))->format($date);
+
+        $this->assertSame($expected, (new IntlExtension($derivedProto))->formatDateTime($env, $date, timezone: 'UTC', locale: 'en_US'));
+
+        $patternProto = new \IntlDateFormatter('fr_FR', \IntlDateFormatter::LONG, \IntlDateFormatter::SHORT, 'UTC', \IntlDateFormatter::GREGORIAN, 'yyyy-MM-dd');
+
+        $this->assertSame('2019-08-07', (new IntlExtension($patternProto))->formatDateTime($env, $date, timezone: 'UTC', locale: 'en_US'));
+
+        // ICU derives the pattern from the calendar of the prototype as well
+        $calendarProto = new \IntlDateFormatter('ja_JP@calendar=japanese', \IntlDateFormatter::LONG, \IntlDateFormatter::SHORT, 'UTC', \IntlDateFormatter::TRADITIONAL);
+
+        $this->assertSame($expected, (new IntlExtension($calendarProto))->formatDateTime($env, $date, timezone: 'UTC', locale: 'en_US'));
+    }
+
+    public function testFormatterProtoWithoutDateAndTimeStylesKeepsItsLocale(): void
+    {
+        $env = new Environment(new ArrayLoader());
+        $date = new \DateTime('2019-08-07T23:39:12+00:00');
+        // ICU reports the "root" locale for a prototype with no date and no time style, and rejects it as an input locale
+        $proto = new \IntlDateFormatter('fr_FR', \IntlDateFormatter::NONE, \IntlDateFormatter::NONE, 'UTC', \IntlDateFormatter::GREGORIAN, 'yyyy-MM-dd');
+
+        $this->assertSame('2019-08-07', (new IntlExtension($proto))->formatDateTime($env, $date, timezone: 'UTC'));
+
+        $localeDependentProto = new \IntlDateFormatter('fr_FR', \IntlDateFormatter::NONE, \IntlDateFormatter::NONE, 'UTC', \IntlDateFormatter::GREGORIAN, 'EEEE d MMMM y');
+        $default = \Locale::getDefault();
+        \Locale::setDefault('en_US');
+
+        try {
+            $this->assertSame('mercredi 7 août 2019', (new IntlExtension($localeDependentProto))->formatDateTime($env, $date, timezone: 'UTC'));
+        } finally {
+            \Locale::setDefault($default);
+        }
+    }
+
     public function testDateFormatterCacheIsBounded(): void
     {
         $ext = new IntlExtension();