I’m having the same problem. It’s very glitchy. I do not have to render, however, to preview the effect. Simply clicking off the matte and onto another effect works (most of the times), but when I play it from the timeline, I can see the effect properly.
However, very difficult to work with as sometimes the matte “preview” blacks everything out and you can’t see what you’re masking.
Did you make any headway with this issue?
Peace,
Bud