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.