I've had multiple instances of masks failing to work. Suddenly the mask will just be a path and not mask anymore. restarting AE seems to fix it. Toggling add,substract etc does nothing, attempts to check masks are closed does nothing.
Example screenshot is 2 add masks that aren't working.
Thank you for reporting this issue. This should be fixed in the latest After Effects (Beta) build (24.1.0.56). Please give it a try when you can and let us know if you continue to see this issue.