This effect have been seen in 1.12.2 and 1.15. First noticed on a public server and now on my realm. When players tp to another player who is in the nether, end or different world. the xp bar vanishes, but when you collect any xp it returns to the correct amount. You can use an anvil while it's not showing, but not the enchant table until the xp shows again. I've included 2 ss before and after a tp to show the results.