E-Mail activation doesn't stick with AD authentication

I set up gitea in an lxc container on my proxmox sever, but already had the same problem on a test setup with docker.
I configured my active directory as an authentication source via “LDAP (via BindDN)”, which is working fine for login. On login I need to confirm my e-mail, which is also working, but at the next login I need to confirm my e-mail again. The activation just does not stick. The activation check mark of AD users in the “User Emails”-Tab of the configuration is grey, unlike the green check marks of local users or users authenticated via SMTP.

What am I doing wrong here?

Best regards,