I ran into an issue on 8.4p1 where validation scripts were firing twice causes a lot of issues with existing forms. I was told the fix was targeted for 8.4p2, but maybe it was not fixed and is related to your issue. I found this similar post at the time as well: Field validation happens twice in form(s)
I would add some logging to your scripts and see if you are seeing something similar because you should be able to get an e-fix. I was given one for 8.4p1.