ZDoom

Seems to work fine now.

The prediction strikes again. When unpredicting a player the flags seems to get restored...
That's another of those 'dirty' pieces of code that leave their footprint all over the place and cause problems.

Can you please retry with the latest commit?

The issue has been traced to this commit.

And I thought it was due to network lag... though we played via LAN, so it did seem weird.

Reproduced on Plutonia MAP15.

Steps to reproduce:
.5. Pick your favorite map with a teleporter. We personally reproduced the issue on Heretic E1M1, just flick the switch down the hall with the window to the secret and come back to that big room.
1. Launch 2 GZDoom sessions, one -host 2 and one -join 127.0.0.1.
2. Walk through the teleporter, watch ugly stuttering happen.

When playing in multiplayer with more than one player (tests with "-host 1" will not yield results) the screen will begin to stutter after any player enters any teleporter. The problem goes away if the game is saved after the stuttering begins.