diff --git a/src/Rules/RuleLevelHelper.php b/src/Rules/RuleLevelHelper.php
index 1b46ccfbe7..9c05539935 100644
--- a/src/Rules/RuleLevelHelper.php
+++ b/src/Rules/RuleLevelHelper.php
@@ -95,7 +95,7 @@ private function transformAcceptedType(Type $acceptingType, Type $acceptedType):
 
 				return new CallableType(
 					$acceptedType->getParameters(),
-					$traverse($this->transformCommonType($acceptedType->getReturnType())),
+					$traverse($acceptedType->getReturnType()),
 					$acceptedType->isVariadic(),
 					$acceptedType->getTemplateTypeMap(),
 					$acceptedType->getResolvedTemplateTypeMap(),
@@ -111,7 +111,7 @@ private function transformAcceptedType(Type $acceptingType, Type $acceptedType):
 
 				return new ClosureType(
 					$acceptedType->getParameters(),
-					$traverse($this->transformCommonType($acceptedType->getReturnType())),
+					$traverse($acceptedType->getReturnType()),
 					$acceptedType->isVariadic(),
 					$acceptedType->getTemplateTypeMap(),
 					$acceptedType->getResolvedTemplateTypeMap(),
@@ -142,10 +142,10 @@ private function transformAcceptedType(Type $acceptingType, Type $acceptedType):
 				}
 			}
 
-			return $traverse($this->transformCommonType($acceptedType));
+			return $traverse($acceptedType);
 		});
 
-		return [$acceptedType, $checkForUnion];
+		return [$this->transformCommonType($acceptedType), $checkForUnion];
 	}
 
 	/** @api */
diff --git a/tests/PHPStan/Rules/Classes/InstantiationRuleTest.php b/tests/PHPStan/Rules/Classes/InstantiationRuleTest.php
index 511bf6136d..a1db3f755a 100644
--- a/tests/PHPStan/Rules/Classes/InstantiationRuleTest.php
+++ b/tests/PHPStan/Rules/Classes/InstantiationRuleTest.php
@@ -20,6 +20,8 @@
 class InstantiationRuleTest extends RuleTestCase
 {
 
+	private bool $checkExplicitMixed = false;
+
 	protected function getRule(): Rule
 	{
 		$reflectionProvider = self::createReflectionProvider();
@@ -27,7 +29,7 @@ protected function getRule(): Rule
 		return new InstantiationRule(
 			$container,
 			$reflectionProvider,
-			new FunctionCallParametersCheck(new RuleLevelHelper($reflectionProvider, true, false, true, false, false, false, true), new NullsafeCheck(), new UnresolvableTypeHelper(), new PropertyReflectionFinder(), $reflectionProvider, true, true, true, true),
+			new FunctionCallParametersCheck(new RuleLevelHelper($reflectionProvider, true, false, true, $this->checkExplicitMixed, false, false, true), new NullsafeCheck(), new UnresolvableTypeHelper(), new PropertyReflectionFinder(), $reflectionProvider, true, true, true, true),
 			new ClassNameCheck(
 				new ClassCaseSensitivityCheck($reflectionProvider, true),
 				new ClassForbiddenNameCheck($container),
@@ -570,6 +572,13 @@ public function testBug14097(): void
 		$this->analyse([__DIR__ . '/data/bug-14097.php'], []);
 	}
 
+	#[RequiresPhp('>= 8.0')]
+	public function testBug13440(): void
+	{
+		$this->checkExplicitMixed = true;
+		$this->analyse([__DIR__ . '/data/bug-13440.php'], []);
+	}
+
 	public function testNewStaticWithConsistentConstructor(): void
 	{
 		$this->analyse([__DIR__ . '/data/instantiation-new-static-consistent-constructor.php'], [
diff --git a/tests/PHPStan/Rules/Classes/data/bug-13440.php b/tests/PHPStan/Rules/Classes/data/bug-13440.php
new file mode 100644
index 0000000000..dedd1b08fb
--- /dev/null
+++ b/tests/PHPStan/Rules/Classes/data/bug-13440.php
@@ -0,0 +1,64 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug13440;
+
+use Closure;
+
+/** @template T */
+interface Foo {}
+
+/**
+ * @template TVal
+ * @template TReturn
+ */
+class Box
+{
+    /**
+     * @param TVal $val
+     * @param Closure(Foo<TVal>): TReturn $cb
+     */
+    public function __construct(
+        private mixed $val,
+        private Closure $cb,
+    ) {
+    }
+
+    /**
+     * @template TNewReturn
+     * @param Closure(Foo<TVal>): TNewReturn $cb
+     * @return self<TVal, TNewReturn>
+     */
+    public function test(Closure $cb): self
+    {
+        return new self($this->val, $cb);
+    }
+}
+
+/**
+ * @template TVal
+ * @template TReturn
+ */
+class Box2
+{
+	/**
+	 * @param TVal $val
+	 * @param callable(Foo<TVal>): TReturn $cb
+	 */
+	public function __construct(
+		private mixed $val,
+		private $cb,
+	) {
+	}
+
+	/**
+	 * @template TNewReturn
+	 * @param callable(Foo<TVal>): TNewReturn $cb
+	 * @return self<TVal, TNewReturn>
+	 */
+	public function test($cb): self
+	{
+		return new self($this->val, $cb);
+	}
+}
diff --git a/tests/PHPStan/Rules/Properties/TypesAssignedToPropertiesRuleTest.php b/tests/PHPStan/Rules/Properties/TypesAssignedToPropertiesRuleTest.php
index cf7136dd1f..6c03bc86c3 100644
--- a/tests/PHPStan/Rules/Properties/TypesAssignedToPropertiesRuleTest.php
+++ b/tests/PHPStan/Rules/Properties/TypesAssignedToPropertiesRuleTest.php
@@ -1025,6 +1025,14 @@ public function testBug12250(): void
 		$this->analyse([__DIR__ . '/data/bug-12250.php'], []);
 	}
 
+	#[RequiresPhp('>= 8.0')]
+	public function testBug12688(): void
+	{
+		$this->checkExplicitMixed = true;
+		$this->checkImplicitMixed = true;
+		$this->analyse([__DIR__ . '/data/bug-12688.php'], []);
+	}
+
 	public function testBug4525(): void
 	{
 		$this->analyse([__DIR__ . '/data/bug-4525.php'], []);
diff --git a/tests/PHPStan/Rules/Properties/data/bug-12688.php b/tests/PHPStan/Rules/Properties/data/bug-12688.php
new file mode 100644
index 0000000000..6d3b01c64b
--- /dev/null
+++ b/tests/PHPStan/Rules/Properties/data/bug-12688.php
@@ -0,0 +1,54 @@
+<?php // lint >= 8.1
+
+namespace Bug12688;
+
+/**
+ * @template T = mixed
+ */
+interface I {}
+
+/**
+ * @implements I<mixed>
+ */
+enum E implements I
+{
+	case E;
+}
+
+/**
+ * @template T
+ */
+final class TemplateWithoutDefaultWorks
+{
+	/**
+	 * @var I<T>
+	 */
+	public readonly I $i;
+
+	/**
+	 * @param I<T> $i
+	 */
+	public function __construct(I $i = E::E)
+	{
+		$this->i = $i;
+	}
+}
+
+/**
+ * @template T = mixed
+ */
+final class TemplateWithDefaultDoesNotWork
+{
+	/**
+	 * @var I<T>
+	 */
+	public readonly I $i;
+
+	/**
+	 * @param I<T> $i
+	 */
+	public function __construct(I $i = E::E)
+	{
+		$this->i = $i;
+	}
+}