If you a runing plugin that uses the /up command for building it can be exploited.  If you use the command and don't have resources it won't upgrade but still gives you XP.

So it's another plugin conflict? 

Not really a conflict just a plugin that auto upgrades your building if you have the mats on you.  If you don't it tells you no and how much you need..  The Xperience plugin reads this as if they upgraded and awards XP.

https://umod.org/plugins/building-grades

Ok, I'll see if I can get it to detect this and prevent the xp gains