So I was in a survival world 1.16.5 Optifine was the only mod. I hadn't died once in the world or spent the XP but it disappeared I had 30 levels last I remember I have no idea what happened to it so I'm assuming it was a bug.
MC-219141
My XP disappeared without me dying or spending it.
Resolved
Invalid
0
Because you are using Optifine, I'm sorry to say that we can't accept it. If you experience the bug in a vanilla environment, feel free to create another report.