I'm playing in JJ2+ version (so the one which should be better adjusted for multiplayer). By some reason, when playing in multiplayer, I've got constantly issue with other players flickering/disappearing/slowed animation characters. The weird thing is, that even when I've played with my sister not on internet, but on LAN, the problem still occured. I don't think it's because my internet/network connection speed, but I don't have a clue what other reason could cause it.

I have also set adequate changes in firewall settings for Jazz, so it also shouldn't be a problem. Yet.. It's still there.

Players flicker whenever they get hurt and lose a heart, but I suppose this is not what you're talking about. If you join the jj2 discord and maybe video capture your game issue, then I'm sure people are willing to help you.