Since a song is defined a set number of bars or measures, then restarting a part will change the length of a song.
For example, if you’re playing with a group and you hit “fill”, either on purpose or by accident while trying to hit something else, then the entire song is screwed. Especially since you won’t know exactly how many measures are now added to a song.
Feature request is for Fills to act just as if you set them in the BBMO, just play the fill over the part and continue so the song length and other transitions aren’t affected.