Someone removes added users from groups

If I add some user to some group after some time (about an hour) user is removed from group. User recieves a mail that I added him to group and that administrator removed him from it. But there’s no such user as administrator. All users added to groups earlier (more than a year) are in groups.

Nextcloud version (eg, 20.0.5): 20.0.10
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.41
PHP version (eg, 7.4): 7.4.3

The issue you are facing:

Is this the first time you’ve seen this error? (Y/N): N

Steps to replicate it:

  1. Add user to group

The output of your Nextcloud log in Admin > Logging:
There’s no anything about it