Players fly, get invisible, duplicate, when coming trough a portal

I play on a multiplayer server, and when players come trough a portal, they become invisible or it looks like they fly, or they duplicate, i don't think it's server lagg, because its a small server with +/- 5 people maximum online at the same time, and we haven't got any server lagg before, so i thought i would report it as a bug
And it solves itselves when one of the 2 players relogs

EDIT : When a player enters the nether through a portal : it looks like the player is under the floor(the floor is made from half slabs and there is air beneath the half slabs) And when a player enters the overworld through a portal, thats when the player becomes invisible or gets duplicated