Mod version 0.8.0.9
Launcher is Curseforge, 1.20.1

Relics don't comprehend storing xp with an external mod, only allowing xp to be altered "organically" (via xp orbs or, probably, commands). Tested with both Sophisticated Backpacks xp pump upgrade with a tank in an iron backpack and the Improvable Skills xp storage. I know this wasn't the case on 1.19, at least not with the xp storage I used on that version, so I don't know quite what broke, but it's clunky, allows re-rolling once for free, and then bricks it unless you get xp orbs normally

I'm assuming it's on Relics end since it doesn't work with either Sophisticated Backpacks or Improvable Skills and worked on the 1.19 version, though it could be caused by some other mod. I also assume it has something to do with how Relics checks for xp updates compared to prior versions