Player will randomly become invisible on servers.

Even if they are wearing armor the other players can't see them. Particles generated by the invisible players (fall damage, hearth, potion effects, etc.) are invisible too.

When you are affected by the bug, you can't see your armor points and you don't any of your sounds (fall damage, taking damage, burning, etc.). You are also immune to melee damage dealt by other players, but you still take damage from arrows although you don't get knocked back. Spectators can not teleport to invisible players and boats used by invisible players will act strangely.

The only known way to fix it is by having the player to relog or kill him.

The bug is about chunk loading on a server. If a player gets teleported in an unloaded chunk, he will become invisible most of the time. If you teleport another player in the same chunks as the first one, he won't be invisible because the chunks are loaded by the first player. If you teleport a player at least 1 tick in the spawn chunks and then teleport them somewhere else they won't become invisible. This could be used as a temporary fix for map-makers.

https://www.youtube.com/watch?v=NLEH_Mzcm6w
https://youtu.be/RL6pmNHjPfo