[Ben Johnson]

It is indeed the edge of the original frame, and I do have a crop applied to trim that corner (see second screenshot).

It’s not there permanently, it flickers on and off. When I’m editing in the timeline I can see it sometimes (mainly when editing titles) but literally any change within Premiere will make it disappear. Selecting a different layer, selecting a different tool, solo an audio track, and boom: it’s gone.

That lead me to believe it’s just a timeline playback issue and an exported version wouldn’t contain the flickering corner, but alas, it pops in and out of the exported video as well.