The instance of your script created thought the autoload mechanism is a direct child of the "root" node in the SceneTree. Your tree looks like this when the game is running:
To better see what is happening, start your game, then focus the Editor without closing the game, and just above the tree view on your left, select "Remote". This will show you the tree of your game as it is running. You can also call
$"/root".print_tree_pretty() (inside some Node's _ready()) for a textual representation.
To access your Button, you can use:
- Direct tree walking, with $"/root/MyScene/UI/UIbackground/Uibackground/VBoxContainer/TextureButton", or get_node() with the same path.
- Have the Button (or someone that has an easy access to it), register it to your Autoload. That would look like
MyAutoload.sacrificial_button = self inside the Button's script, and
if is_instance_valid(sacrificial_button): sacrificial_button.queue_free() inside the Autoload.