I don't know how to reproduce it, but I keep occasionally noticing that buildings have lost the priority I gave them and have gone back to priority five after reloading the game. One time my outhouses had lost their priority which caused the dupes to stop maintaining them causing a bunch of messes to be made. Just now I noticed that my bottle emptier had priority five after I had given it priority seven or eight. Now that I think about it I think there've been more cases where I thought "Hey hadn't I given that building a priority?" but assumed that I must've forgotten to give it one.
Steps to Reproduce
I'm not sure, perhaps repeatedly saving and loading a save file and checking the priorities every time might work.
This issue has not been confirmed by a developer yet.