https://clipchamp.com/watch/ZXYL3Lbz4IH

This one seems to be a little inconsistent as you'll see in the clip above it took a few tries, but after mega evolving (in this case Gardevoir) its ability will change from Trace to Synchronize after reverting to its non-mega form.

Greetings,

I'm afraid I couldn't repeat this issue. I tried with both Gardevoir and Slowbro (both using their second listed abilities) with 10 battles each.

Was there any plugins or mods that would affect abilities or mega forms installed?

While the previously linked clip was on a live server with other plugins and sidemods, I can consistently recreate it on singleplayer with only pixelmon installed, and no datapacks. Clip and log attached were even on a newly created world, with freshly regenerated configs. Notice that you can see it convert from its regular form back into a mega form after the battle has ended (right before I summon the bulbasaur look in my party, as well as while im inspecting the pokemon UI near the end). When this happens, I know the ability has broken.

New recording from singleplayer: https://clipchamp.com/watch/eg2QzMYdu87
Client log: https://mclo.gs/byghVDa

While re-testing this I've also had Pixilate remain stuck on Gardevoir after it's reverted to non-mega and stayed like that.

Was able to finally able to reproduce it slightly differently.

Thanks!!