Found another Godot crime: https://github.com/godotengine/godot/issues/69014

It looks like lambdas do not capture local variables as they only copy the value of each variable. However, since Variant is passed by reference, you can use a Variant to get around this problem.

For example, this does not work:

func foobar():
	var stop = false
	var stop_callback = func():
			stop = true
	some_signal.connect(stop_callback)

	await get_tree().create_timer(10).timeout

	print(stop) # Always false

But this does:

func foobar():
	var stop = { "value": false }
	var stop_callback = func():
			stop["value"] = true
	some_signal.connect(stop_callback)

	await get_tree().create_timer(10).timeout

	print(stop["value"]) # Either true or false