Invisible entities in Minecraft multiplayer

I was playing with a friend in a multiplayer world. He died and I teleported him back.

*I couldn't see him and I couldn't interact with him, but my friend could see itself and me.

*

I tried to reload chunks but nothing changed.

Later, I re-joined the server, and I was again able to see him.

After some minutes of travelling (in a Nether Fortress), an invisible Wither Skeleton hit us, but again we weren't able to see or interact with him, so we died.

When we respawned, my friend couldn't see or interact with me, but I could.

I don't know what this bug is, I'm sure that this bug isn't related to the Internet connection (because it happened to both of us).

I think this bug is related to the host platform of the server (Aternos.me), but I'm not sure of that.

(Server was running at about 20 TPS)