fix: revert "prevent prematurely cloning light dom slot vnodes"

This reverts commit 1105823 (#4258).

packages/@lwc/engine-core/src/framework/api.ts

@@ -281,15 +281,7 @@ function s(
                         // to the vnode because the current way the diffing algo works, it will replace the original reference
                         // to the host element with a new one. This means the new element will be mounted and immediately unmounted.
                         // Creating a copy of the vnode to preserve a reference to the previous host element.
-                        if (isUndefined(vnode.elm)) {
-                            // vnode.elm is undefined during initial render.
-                            // We don't need to clone at this point because it doesn't need to be unmounted.
-                            vnode.slotAssignment = data.slotAssignment;
-                        } else {
-                            // Clone when the vnode.elm is defined to ensure we don't lose reference to the previous element.
-                            // This is specifically for slot forwarding.
-                            clonedVNode = { ...vnode, slotAssignment: data.slotAssignment };
-                        }
+                        clonedVNode = { ...vnode, slotAssignment: data.slotAssignment };
                     }
                     // If the slot content is standard type, the content is static, no additional
                     // processing needed on the vnode