After making my Zigzagoon headbutt the tree in the old guy's garden repeatedly to the point that it has now brain damage, I finally got my Bidoof to 15 aaaaand.... *fart noise*
It didn't evolve, tried leveling it up again and the same thing happened.
Latest Patreon version of Joiplay, Reborn 19.16, Debug Mode enabled (though I don't think this is the culprit, since my Torchic evolved into Combusken just fine)