diff --git a/src/Analyser/MutatingScope.php b/src/Analyser/MutatingScope.php
index a26a86a066..4ef3c14e30 100644
--- a/src/Analyser/MutatingScope.php
+++ b/src/Analyser/MutatingScope.php
@@ -88,6 +88,7 @@
 use PHPStan\Reflection\Php\PhpMethodFromParserNodeReflection;
 use PHPStan\Reflection\PropertyReflection;
 use PHPStan\Reflection\ReflectionProvider;
+use PHPStan\Reflection\Type\UnionTypePropertyReflection;
 use PHPStan\Rules\Properties\PropertyReflectionFinder;
 use PHPStan\ShouldNotHappenException;
 use PHPStan\TrinaryLogic;
@@ -4897,12 +4898,32 @@ public function canAccessProperty(PropertyReflection $propertyReflection): bool
 	/** @api */
 	public function canReadProperty(ExtendedPropertyReflection $propertyReflection): bool
 	{
+		if ($propertyReflection instanceof UnionTypePropertyReflection) {
+			foreach ($propertyReflection->getProperties() as $innerProperty) {
+				if (!$this->canReadProperty($innerProperty)) {
+					return false;
+				}
+			}
+
+			return true;
+		}
+
 		return $this->canAccessClassMember($propertyReflection);
 	}
 
 	/** @api */
 	public function canWriteProperty(ExtendedPropertyReflection $propertyReflection): bool
 	{
+		if ($propertyReflection instanceof UnionTypePropertyReflection) {
+			foreach ($propertyReflection->getProperties() as $innerProperty) {
+				if (!$this->canWriteProperty($innerProperty)) {
+					return false;
+				}
+			}
+
+			return true;
+		}
+
 		if (!$propertyReflection->isPrivateSet() && !$propertyReflection->isProtectedSet()) {
 			return $this->canAccessClassMember($propertyReflection);
 		}
diff --git a/src/Reflection/Type/UnionTypePropertyReflection.php b/src/Reflection/Type/UnionTypePropertyReflection.php
index 33eb2a6824..76dbc28905 100644
--- a/src/Reflection/Type/UnionTypePropertyReflection.php
+++ b/src/Reflection/Type/UnionTypePropertyReflection.php
@@ -23,6 +23,14 @@ public function __construct(private array $properties)
 	{
 	}
 
+	/**
+	 * @return ExtendedPropertyReflection[]
+	 */
+	public function getProperties(): array
+	{
+		return $this->properties;
+	}
+
 	public function getName(): string
 	{
 		return $this->properties[0]->getName();
diff --git a/tests/PHPStan/Rules/Properties/AccessPropertiesRuleTest.php b/tests/PHPStan/Rules/Properties/AccessPropertiesRuleTest.php
index e5e2edc4d8..ba60307a93 100644
--- a/tests/PHPStan/Rules/Properties/AccessPropertiesRuleTest.php
+++ b/tests/PHPStan/Rules/Properties/AccessPropertiesRuleTest.php
@@ -1246,4 +1246,12 @@ public function testBug13537(): void
 		$this->analyse([__DIR__ . '/data/bug-13537.php'], $errors);
 	}
 
+	public function testBug12280(): void
+	{
+		$this->checkThisOnly = false;
+		$this->checkUnionTypes = true;
+		$this->checkDynamicProperties = false;
+		$this->analyse([__DIR__ . '/data/bug-12280.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Properties/data/bug-12280.php b/tests/PHPStan/Rules/Properties/data/bug-12280.php
new file mode 100644
index 0000000000..0441ae7379
--- /dev/null
+++ b/tests/PHPStan/Rules/Properties/data/bug-12280.php
@@ -0,0 +1,53 @@
+<?php // lint >= 8.2
+
+declare(strict_types = 1);
+
+namespace Bug12280;
+
+final readonly class A
+{
+	public function __construct(
+		public \DateTime $date,
+	) {}
+}
+
+class B
+{
+	public function __construct(
+		private \DateTime $date,
+	) {}
+
+	/**
+	 * @param list<A> $a
+	 * @param list<B> $b
+	 * @return list<\DateTime>
+	 */
+	public static function test1(array $a, array $b): array
+	{
+		$getDate = static function(A|self $value): \DateTime {
+			return $value->date;
+		};
+
+		return [
+			...array_map($getDate(...), $a),
+			...array_map($getDate(...), $b),
+		];
+	}
+
+	/**
+	 * @param list<A> $a
+	 * @param list<B> $b
+	 * @return list<\DateTime>
+	 */
+	public static function test2(array $a, array $b): array
+	{
+		$getDate = static function(self|A $value): \DateTime {
+			return $value->date;
+		};
+
+		return [
+			...array_map($getDate(...), $a),
+			...array_map($getDate(...), $b),
+		];
+	}
+}