So many years and we still don't have a proper way to Mask without bugs

The logic behind the mask layers of this software is bogus.

Aside from that, they work with Vibes only. Basically, sometimes they do, sometimes they don't.

Please tell me what I'm doing wrong.

I just duplicated the layer and put a mask on it so that I can draw shadows on it. It works 80% of the time, aside from when Animate bugs. Is it because the layers have objects inside???

GIF:

I hate this software. I wish I didn't grow with it. I wish there was an alternate.