ok so i finishe building my enderman farm and i used it for a while and got to level 30 so then i /home tp away from the enderman farm to go to apply mending on my items but i couldnt because my EXP was gone it was drained to 0 so i went back to the farm again and my exp still didnt appear so i killed a few endermen and finally my exp showed up but then i warped away and lost my exp again it doesnt matter how many times i try the exp will vanish once i tp away.

I think this is an MC bug. Lmk if this solves your problem: #128 (comment)

thank you