I just encountered this a few minutes ago. I was setting up my universe test place for my game, and I came across something that was really messing with my head. Whichever game I set as the start place turns out looking bugged. You can still play with a direct link, but it also doesn’t show in group games.
This gif shows what I’m saying.
Did you enable the setting in Group Admin which shows group games on the group’s page?
My friend Repotted just told me that.
But now the universe start place appears as this
https://gyazo.com/8e2df5175011d9356c595eaab8197b7d
EDIT
What it looks like in the dev page if anyone is interested
https://gyazo.com/16dc7dd47f229ae9a85656228bc0568f
We were able to resolve the group games thumbnail issue by removing/re-adding the places, and we can’t reproduce it anymore.
Start Place: It's not what I think it is - Roblox
Sub Place: https://www.roblox.com/games/1016241095/Some-kind-of-lobby
GameId: 421150267