EDIT: I originally only tried switching between the last 2 proton versions and that didn't work. But then I realized that last weekend, when it last worked, the current one was actually ge-proton 10-6. Switched back to that and it seems to have fixed it. So, some ge-proton update in between apparently doesn't play nice with game mode. Should have thought about that from the start.
Thanks to all who replied.
Original post: On my steam deck OLED I have several GOG games installed via heroic and added to steam via heroic's built in function. Those games were working fine last weekend.
I was going to play tonight but starting from game mode, my non-steam games are now stuck in an infinite loading screen (spinning steam logo) but with their audio playing in the background. As far as I can tell I didn't change anything in between, but there might have been an update installed after my last gaming session, I'm not sure.
The main one I am trying to get to work again is Star Renegades. I also tested with The Legend of Heroes: Trails Through Daybreak with the same outcome.
Steam games seem to work normally.
Affected games work just fine in desktop mode, from heroic as well as the desktop steam app. But as I mainly play in docked mode on my TV that's no real alternative (TV isn't recognized outside game mode).
Switching proton versions didn't seem to work, neither did changing from full screen to windowed, rebooting the steam deck multiple times and re-adding the game to steam.
Heroic, steam, the games, proton and just about anything else I can think of are updated to their current stable versions.
I found some posts on the internet suggesting it had to do with the startup animation settings, but those are all switched off and I never touched them.
Does anybody have any ideas what else to check?