Simple way to reproduce this bug -> you have to be in minecraft server :
to be in survival mode
/worldborder center 0 0
/worldborder set 2001
/worldborder set 201 7200 (to reduce slowly the map to 201 during 2h)
-> If one player disconnect and reconnect to the server during the reduction of map, the worldborder is reduced to 201 immediatly for this player ONLY, and the player can't move anymore but will not suffocate.
For the others players, things seems normal.
It works even if there is only one player.
Please don't put down the affected version as a future version. You DON'T have access to it yet!