Vocal parts' volumes drifting

My score is having an issue where, as the song goes on, some of my parts seem to be randomly drifting louder or quieter as the song goes on. The notation is a little messy since it's incomplete, but here's the song where the bug is happening: https://musescore.com/user/90618763/scores/24204211/s/A2giQW?share=copy…

If you listen to the beginning, the parts are well balanced. But gradually (and if you go to the end) you'll hear, for example, that the Bass is very quiet and the Alto is way too loud. This is in spite of the fact that I made sure the velocities of their notes in playback, dynamic markings, and their values on the mixer are equal to the other parts. Has anyone here had this issue?