I’ve noticed two anomalies with using effects masks. If we agree that they are bugs, and they haven’t already been reported, I can try reporting them.
1 – When you apply a mask to an effect, it is activated for all the current effects for the clip, but only one mask icon is highlighted. Switch away and back to refresh the display and all the icons are shown as active.
2 – If you add a mask to a clip which has something on the track below, but not starting at the begins of the clip, the mask only extends to the next object. I would have though it should be put on a track that is completely empty for the duration of the target clip