Oh I just tested again and it turns out it actually respawn the objective item back to the capture Item spawner once it is removed by a mutator zone. But If it’s a regular item spawner, the item is removed completely from the game.
It’d still be nice to be able to specify items that shouldn’t be affected by the mutator zone, so an objective item can remain exactly where it is dropped to avoid the risk of confusing players or worse, return back to a location players are unable to get back to, but it helps to know that the capture item spawner ensure items don’t leave the game.