Seemingly was able to fix this by manually setting my computer's time to 1:30pm. When I was doing it before it was between 4 and 5 pm. I don't know if this is what was causing the issue or not. In any case it's working now. The successful evolution happened against the Grand Hall trainer who leads with Drifloon.
If you're still interested, I tried it in multiple encounters. One was a Grand Hall trainer battle (the one with the ninja turtles), one was random trainer in North Obsidia, and I also tried loading an older save and evolving against Corey. Tried evolving at levels 34 and 35. None of this worked. Possibly relevant: rather than downloading the patch to play the mod, I downloaded the entire All Gen Reborn folder (this one:
https://drive.google.com/file/d/1hiqH_pTE9SVIHJ8y289GD25HOEUgEi0E/view?usp=share_link) and launched from there, despite already having Reborn installed, so maybe it was reading from the wrong folders?
