[update] save states don't seem to be working in this mode.
After the new update, Sonic 3 and Knuckles isn't working from the room hub when you try to load the game. It has either stayed a blank screen or crash the game, and i'm sure this is happening for other games. If you want to avoid waiting for SEGA to fix the problem, there is an easy work around to load any rom you like in the game hub. This isn't a total guide but should help some people out.
1: go to steamapps\workshop\content\34270 in your steam folder on windows. If you need help determining this, try google, mine is on a different hard drive.
2: Download any mod from the workshop. I would suggest Sonic 1 since mods are currently working for that game and there are a ton of them avalible.
3: go to folder 34270 in the workshop content folder shown in step 1. Go to the most recently added file (windows will show the most recently moddified folder to the right of the folder name) and open that folder.
4: Delete the .bin file and in it's place copy/paste any genesis rom you want to use into this folder. Make sure the rom is a .bin file, if it is any other type of file rename it to a .bin
5: Boot up the room hub, select the mod you downloaded in step 2 from the mods option, and load this mod. Now the rom used in step 4 will load instead!
If you are experiencing problems with this method, please comment below or reach me direclty to troubleshoot. Hope this helps anyone else out.