I found a different workaround:

1. Host a coop game and start the map
2. hit F10 and click Rehost Game
3. start the map again
4. close the game (I use f10 > Exit to Windows)

After this I can always make a custom game, I think because Rehost Game has a strange interaction with mods (it shows all non-UI mods as disabled but when I check in the mod list nothing's changed). There's another way which involves manually disabling ALL (including UI) mods from the coop lobby and then closing the lobby, but I have ~30 mods I use and deselecting all of them is annoying, hence why I use Rehost Game.

can confirm it works for me. Managing active mods in the mod tab and hosting custom games.

can confirm it works for me. Managing active mods in the mod tab and hosting custom games.

I found a different workaround:

1. Host a coop game and start the map
2. hit F10 and click Rehost Game
3. start the map again
4. close the game (I use f10 > Exit to Windows)

After this I can always make a custom game, I think because Rehost Game has a strange interaction with mods (it shows all non-UI mods as disabled but when I check in the mod list nothing's changed). There's another way which involves manually disabling ALL (including UI) mods from the coop lobby and then closing the lobby, but I have ~30 mods I use and deselecting all of them is annoying, hence why I use Rehost Game.