My second account randomly teleporting up in the air randomly for 1 frame in creative in lan multiplayer on integrated server. No addons. No mods. This happened to me in 1.20.80 and 1.21.60. I didn’t find anything online about this. This might be rendering issue or render optimization issue to be more specific.
It’s very hard to reproduce this, but first you need to put second account far away (maybe behind something) so it needs to be far left from your PoV. Try flying around until you see it
How far away is the second player from the platform where they appear? Are any of the command blocks running teleport commands?