1920x1040 → 3840x2080 rhea (all settings default)
1 pause during processing + 1 resume
Result:
part1.mkv - contains duplicate fame at the end
part2.mkv - contains duplicate frame at start
result.mkv - contains just a copy of part2.mkv
So I was forced to join part1 and 2 using reencode and drop 2 frames in the middle.