Could be a bug. To verify, it would be the best to check if it happens on well defined setups as well, e.g. the test setups (Demo instances of 9, 10, 11 and 12 now live). If it confirms the suspicion, you can file the bug at the bug tracker. There you see, that there are already a few quota-related reports:
https://github.com/nextcloud/groupfolders/issues/215
https://github.com/nextcloud/groupfolders/issues/217