So here's the situation:
I created a png of a text event and have it on a track set to "compositing mode multiply " to have the negative text effect with a video underneath shining through the empty text.

All good, works fine. I have 4 of these in total. Now this happens: Only the first one gets cut off to black at alwas the exact same spot, although nothing at all happens anywhere else. Then again, it sometimes plays OK, but in the render, it's always cut off.

This doesn't happen wirh any of the other ones. So I tried muting track after track above to see if anything was interfering with that process, and what I found was this:

The uppermost track has a masked black generated media to simulate the 1.85:1 aspect ratio black bars. When I mute that track, it plays fine and the png event isn't cut off anymore.

But it gets weirder:

I then exported the png event as an intermediate prores 422 of all the 4 composited events to avoid any potential conflict with the compositing mode.But now, when exporting the whole timeline, Vegas just creates a black where the prores event is. Again, only the first one.

I've never seen anything like this. Does anyone have an idea what I'm missing or what is going on?

Thanks!