diff --git a/src/Analyser/MutatingScope.php b/src/Analyser/MutatingScope.php
index 8d43a7d6137..9ad4d75e88f 100644
--- a/src/Analyser/MutatingScope.php
+++ b/src/Analyser/MutatingScope.php
@@ -3348,7 +3348,18 @@ public function filterBySpecifiedTypes(SpecifiedTypes $specifiedTypes): self
 	public function addConditionalExpressions(string $exprString, array $conditionalExpressionHolders): self
 	{
 		$conditionalExpressions = $this->conditionalExpressions;
-		$conditionalExpressions[$exprString] = $conditionalExpressionHolders;
+		// Merge rather than overwrite: multiple independent holders can target the same
+		// expression (e.g. `$xIsA = $x instanceof A && $y instanceof A` stores a holder
+		// for `$x` keyed on `$xIsA`; later `$yIsA = $y instanceof A && $x instanceof A`
+		// stores another holder for the same target `$x` keyed on `$yIsA`). Replacing
+		// the existing entry here would throw away the earlier binding, breaking
+		// narrowing inside later `if ($xIsA) { … }` inside `if ($xIsA || $yIsA)`.
+		// Holder keys (`getKey()`) disambiguate identical entries so we still dedupe.
+		$existing = $conditionalExpressions[$exprString] ?? [];
+		foreach ($conditionalExpressionHolders as $holder) {
+			$existing[$holder->getKey()] = $holder;
+		}
+		$conditionalExpressions[$exprString] = $existing;
 		return $this->scopeFactory->create(
 			$this->context,
 			$this->isDeclareStrictTypes(),
diff --git a/tests/PHPStan/Analyser/nsrt/bug-12677.php b/tests/PHPStan/Analyser/nsrt/bug-12677.php
new file mode 100644
index 00000000000..04c955be2db
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-12677.php
@@ -0,0 +1,46 @@
+<?php
+
+namespace Bug12677;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+	public function getBool(): bool
+	{
+		return true;
+	}
+}
+
+function test1(?Foo $foo): void
+{
+	$hasFoo = $foo !== null;
+
+	$bool = $foo?->getBool() ?? false;
+
+	if ($hasFoo) {
+		assertType(Foo::class, $foo);
+	}
+}
+
+function test2(?Foo $foo): void
+{
+	$hasFoo = $foo !== null;
+
+	$bool = $foo !== null ? $foo->getBool() : false;
+
+	if ($hasFoo) {
+		assertType(Foo::class, $foo);
+	}
+}
+
+function test3(?Foo $foo): void
+{
+	$hasFoo = $foo !== null;
+
+	$bool = $hasFoo ? $foo->getBool() : false;
+
+	if ($hasFoo) {
+		assertType(Foo::class, $foo);
+	}
+}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-7716.php b/tests/PHPStan/Analyser/nsrt/bug-7716.php
index fca0a7cfd8a..63280c3b466 100644
--- a/tests/PHPStan/Analyser/nsrt/bug-7716.php
+++ b/tests/PHPStan/Analyser/nsrt/bug-7716.php
@@ -15,7 +15,7 @@ public function sayHello(array $array): int
 		$hasBar = isset($array['bar']) && $array['bar'] > 1;
 
 		if ($hasFoo) {
-			assertType('array{foo?: int, bar?: int}', $array);
+			assertType('array{foo: int, bar?: int}', $array);
 			assertType('int<2, max>', $array['foo']);
 			return $array['foo'];
 		}
@@ -44,7 +44,7 @@ public function sayHello2(array $array): int
 		}
 
 		if ($hasBar) {
-			assertType('array{foo?: int, bar?: int}', $array);
+			assertType('array{foo?: int, bar: int}', $array);
 			assertType('int<2, max>', $array['bar']);
 			return $array['bar'];
 		}
diff --git a/tests/PHPStan/Analyser/nsrt/multi-assign.php b/tests/PHPStan/Analyser/nsrt/multi-assign.php
index ab4e4de1cf8..6a2b4a5f645 100644
--- a/tests/PHPStan/Analyser/nsrt/multi-assign.php
+++ b/tests/PHPStan/Analyser/nsrt/multi-assign.php
@@ -55,12 +55,12 @@ function (bool $b): void {
 function (bool $b): void {
 	$foo = $bar = $baz = $b;
 	if ($bar) {
-		assertType('bool', $b);
+		assertType('true', $b);
 		assertType('bool', $foo);
 		assertType('true', $bar);
 		assertType('bool', $baz);
 	} else {
-		assertType('bool', $b);
+		assertType('false', $b);
 		assertType('bool', $foo);
 		assertType('false', $bar);
 		assertType('bool', $baz);
@@ -70,12 +70,12 @@ function (bool $b): void {
 function (bool $b): void {
 	$foo = $bar = $baz = $b;
 	if ($baz) {
-		assertType('bool', $b);
+		assertType('true', $b);
 		assertType('bool', $foo);
 		assertType('bool', $bar);
 		assertType('true', $baz);
 	} else {
-		assertType('bool', $b);
+		assertType('false', $b);
 		assertType('bool', $foo);
 		assertType('bool', $bar);
 		assertType('false', $baz);
diff --git a/tests/PHPStan/Analyser/nsrt/or-condition-ternary-narrowing.php b/tests/PHPStan/Analyser/nsrt/or-condition-ternary-narrowing.php
new file mode 100644
index 00000000000..64372d8567a
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/or-condition-ternary-narrowing.php
@@ -0,0 +1,63 @@
+<?php
+
+namespace OrConditionTernaryNarrowing;
+
+use function PHPStan\Testing\assertType;
+
+class A {}
+class B {}
+
+/**
+ * Mirrors the shape used in TypeCombinator::intersect() where
+ * `$constArrayIsI` / `$constArrayIsJ` pair up with `$constArray` /
+ * `$otherArray` picked via ternaries inside a nested for-loop over an
+ * `$types` list that gets spliced during iteration.
+ *
+ * @param list<A|B> $types
+ */
+function loopWithTernary(array $types): void
+{
+	$typesCount = count($types);
+	for ($i = 0; $i < $typesCount; $i++) {
+		for ($j = $i + 1; $j < $typesCount; $j++) {
+			$iIsA = $types[$i] instanceof A && ($types[$j] instanceof A || $types[$j] instanceof B);
+			$jIsA = $types[$j] instanceof A && ($types[$i] instanceof A || $types[$i] instanceof B);
+
+			if ($iIsA || $jIsA) {
+				$a = $iIsA ? $types[$i] : $types[$j];
+
+				// `$a` is definitely A: when `$iIsA` holds, the ternary picks
+				// `$types[$i]` which is A; when `$iIsA` is false, the outer
+				// OR forces `$jIsA` so the ternary picks `$types[$j]` which
+				// is A.
+				assertType(A::class, $a);
+			}
+		}
+	}
+}
+
+/**
+ * Same pattern but with plain variables — each `$xIsA`/`$yIsA` records two
+ * narrowings (both `$x` and `$y`). The earlier holder (from the first
+ * assignment) must survive the second assignment so that inside the
+ * outer `if ($xIsA || $yIsA)` the nested `if ($xIsA)` can still fire the
+ * conditional holders attached to `$xIsA`.
+ *
+ * @param A|B $x
+ * @param A|B $y
+ */
+function twoStoredAndNarrowings($x, $y): void
+{
+	$xBothA = $x instanceof A && $y instanceof A;
+	$yBothA = $y instanceof A && $x instanceof A;
+
+	if ($xBothA || $yBothA) {
+		if ($xBothA) {
+			assertType(A::class, $x);
+			assertType(A::class, $y);
+		} else {
+			assertType(A::class, $x);
+			assertType(A::class, $y);
+		}
+	}
+}