Area2D doesn't update its overlapping areas when monitorable changes in the areas it's overlapping

[MightyPrinny]

Godot version:
3.1 alpha2 mono x64

Issue description:
An area that is monitoring will never detect that an area exited if the other area leaves while it is not monitorable and will remain in the list returned by get_overlapping_areas. Similarly an area that is monitoring will never detect that an area entered if the other area enters while it is not monitorable and switches to monitorable while it's inside.
*Steps to reproduce:
This can easily be reproduced with a static area switches between monitorable and not monitorable and another area that checks if get_overlapping_areas is empty

Minimal reproduction project
Area2DProblem.zip

[akien-mga]

Might be a duplicate of #17330, can you confirm that issues happen only when toggling monitorable off, and toggling it on doesn't restore proper behaviour?

[MightyPrinny]

I think it could be the same bug just that the other issue doesn't mention that the list returned by get_overlapping_areas keeps the areas that are not monitorable even if the areas are not overlapping. I wanted to add more details but the way the areas go back to work correctly seems inconsistent.

[akien-mga]

Just a quick update since this issue hasn't got any comment in a while: there are currently two open PRs that aim at solve it (#32299 and #41698), but both are pending review from @reduz.