diff --git a/packages/react-reconciler/src/ReactChildFiber.new.js b/packages/react-reconciler/src/ReactChildFiber.new.js
index 52bd632721184..a92f6a6790d57 100644
--- a/packages/react-reconciler/src/ReactChildFiber.new.js
+++ b/packages/react-reconciler/src/ReactChildFiber.new.js
@@ -342,7 +342,7 @@ function ChildReconciler(shouldTrackSideEffects) {
       const oldIndex = current.index;
       if (oldIndex < lastPlacedIndex) {
         // This is a move.
-        newFiber.flags = Placement;
+        newFiber.flags |= Placement;
         return lastPlacedIndex;
       } else {
         // This item can stay in place.
@@ -350,7 +350,7 @@ function ChildReconciler(shouldTrackSideEffects) {
       }
     } else {
       // This is an insertion.
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
       return lastPlacedIndex;
     }
   }
@@ -359,7 +359,7 @@ function ChildReconciler(shouldTrackSideEffects) {
     // This is simpler for the single child case. We only need to do a
     // placement for inserting new children.
     if (shouldTrackSideEffects && newFiber.alternate === null) {
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
     }
     return newFiber;
   }
diff --git a/packages/react-reconciler/src/ReactChildFiber.old.js b/packages/react-reconciler/src/ReactChildFiber.old.js
index 7313d78f0bf3b..730f08663878c 100644
--- a/packages/react-reconciler/src/ReactChildFiber.old.js
+++ b/packages/react-reconciler/src/ReactChildFiber.old.js
@@ -342,7 +342,7 @@ function ChildReconciler(shouldTrackSideEffects) {
       const oldIndex = current.index;
       if (oldIndex < lastPlacedIndex) {
         // This is a move.
-        newFiber.flags = Placement;
+        newFiber.flags |= Placement;
         return lastPlacedIndex;
       } else {
         // This item can stay in place.
@@ -350,7 +350,7 @@ function ChildReconciler(shouldTrackSideEffects) {
       }
     } else {
       // This is an insertion.
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
       return lastPlacedIndex;
     }
   }
@@ -359,7 +359,7 @@ function ChildReconciler(shouldTrackSideEffects) {
     // This is simpler for the single child case. We only need to do a
     // placement for inserting new children.
     if (shouldTrackSideEffects && newFiber.alternate === null) {
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
     }
     return newFiber;
   }