When a player is added, he is first visible at 0,0,0 and then "transported" to the position given in add player. I believe I've seen this in REALMS too when players travel between dimensions or get killed.

I've tried making the player invisible during that spawn, and it lasts for a tick. So if i make the player invisible shorter than ~50ms it is visible. It appears as if it corrects the position on that first tick.

See the following tweet for a visual of the issue (a bit extreme):
https://twitter.com/NiclasOlofsson/status/841019348462522373

Notice that this was created using 3'rd party server (MINET).