Existing song: add new part which is the first song part. Copy+paste main loop from part 2 into part 1. AP is off. Play the song. The main loop of the first part plays only for a single bar and moves onto the 2nd part.
Enable AP, then disable AP for the 1st part. The part now plays properly (indefinitely).
Existing song: add new part which is the first song part. Copy+paste main loop from part 2 into part 1. AP is off. Play the song. The main loop of the first part plays only for a single bar and moves onto the 2nd part. Enable AP, then disable AP for the 1st part. The part now plays properly (indefinitely).