Unable to pin down the problem; maybe the app doesn't like Readiwip.😉
To work around it, the project was divided into two sequences at 2:30, then those sequences were combined in sequence 3 and exported at 1920 X 1080 - 45fps.
In the future, converting files that are higher than HD res to 1920 X 1080 might help to expedite editing. Quality loss should be negligible.