Permissions broken after upgrade to 8.15

Steps to reproduce

1. Give permissions to group foo to user bar on 8.14
2. Upgrade to 8.15
3. Group foo will still show permissions for user bar on the gitlab web UI

Expected behavior

foo user will actually have permissions.

Actual behavior

foo user will not actually have permissions to do anything re: group foo

Please let me know if the checks are needed, because I'm currently time-constrained - several people have been reporting the same issue on IRC and they could probably help with this.

Workaround

Remove and add permissions to group foo for user bar