Other than the annoying fact that users are now back in groups they don’t want to be in, it is a potential threat to the group owners. Potentially, some private and delicate information may have leaked from some of the closed groups.
Facebook reported that they are aware of this issue and they are now working on a fix. While waiting, you may take action and manually leave all of the groups that you do not wish to be part of. If you own a Facebook group, you should revise which users are granted access and delete unauthorized users from the group.
Now we must wait for Facebook to roll out a bug fix and in the meantime, we can decide whether it is a good idea to share sensitive information even in these closed Groups.