ZDoom

Partially addressed:
https://github.com/coelckers/gzdoom/com ... efe02ca9b2

This needs to be addressed for demos, too, so I am not going to close this just yet. However, Zandronum can safely pull this fix if they're using the new JSON savegame format already.

This is a simple case of the "multiplayer" variable not being serialized with the save game data - nothing more. It should be a simple fix, if I can figure out how it stores this data. I was well aware of this issue for a long time, I just never had the motivation to dive into the save game code and fix it.

It needs to be serialized for demos, too.

This thing was there even before the coop option was added to the map command. I've just confirmed it with 2.2.0.
And with ZDoom LE so it was in ZDoom 2.8.1 as well, probably it's been like that since ever.

Run a new game normally (singleplayer), save it.

If you open the console and run 'map map01 coop' or run gzdoom again in multiplayer mode ('-host 1 -warp 01') and then load the savegame, the game doesn't switch off the multiplayer mode, against what the game was supposed to be when saved. The same is said for viceversa.

I can't check older versions, but I think this bug shouldn't be too old.