call_group("mobs", "queue_free") does not work reliably without waiting a frame (Godot 4)

In the "Your first 2D game" tutorial, mobs are removed using:

get_tree().call_group("mobs", "queue_free")

In Godot 4, this can fail when called in the same frame as game reset (e.g. new_game()),
because both call_group and queue_free are deferred.
As a result, mobs remain in the scene, which is confusing for beginners.

Workarounds that work reliably:
- Waiting one frame:
await get_tree().process_frame
get_tree().call_group("mobs", "queue_free")

- Or directly freeing:
for m in get_tree().get_nodes_in_group("mobs"):
m.free()