I solved the mystery and I’ll post it here (and at at the other forum I posted on) in case anyone else falls down this weird little rabbit hole.
It’s a screen capture project, w/ source footage 1344 x 840. I did the rough edit in a 1344×840 timeline – for timing and sync w/narration. I copied the sequence and converted it to 1280×720 to do the pans, zooms and final framing. From this timeline, I often sent stuff to motion for masks, mattes and graphics – and it worked 85% of the time…
But other stuff was going fuzzy. To fix it, I went back to the 1344×840 timeline – sent to motion from there and rebuilt the shot. Then brought that full rez motion file into 1280×720…
The match cuts between source footage and motion files are now invisible.