I'm having the same issue. It just started last night.
My Flow relies on a Microsoft Form I created and then transferred to a Group that I own. I can see the front end of the Form but not the back end anymore. When I click on it from my recent forms, it gives me an error "Something went wrong. Please make sure you have permission to access the Form." The Group in Forms doesn't show any forms at all.

When I first click on the Group from the Forms web app, it shows a random number of members that doesn't match how many members should be (or actually are) in the Group and when I refresh the Group page, it shows "0 members." But when I click on the members link, it takes me to OWA and shows all 21 members with me listed as the Owner.

All I know to do is try to get another Owner from the Group to try to access the Form, but I have my doubts that will work. Any other ideas?