Thank you for the link to the similar thread. I’ve found that I must start selecting ‘Alpha Add’ in the video settings for each ProRes 4444 clip on the timeline, which fixes the problem with transparency/alpha.
However, it’s inconsistent as another video transition that I just exported worked automatically as soon as I brought it back into FCPX (without me having to set the composite mode to Alpha Add). It’s strange that this has a) suddenly become an issue, and b) is an inconsistent one.
Thank again.