Indestructible xporbs

I was had slain a few sheep in creative mode, and shortly after returning to the world the xp orbs were still there, but I couldn't collect one, but it came towards me and floated up and down, so I attempted to burn it in lava. It didn't disappear so I dug a hole and let it fall into the void, but it didn't disappear, and it landed on the invisible floor. Gamerule keepinventory was on, so I stood on top of it and changed to survival and when I came back, it was next to the hole like nothing happened.