Is it possible that the db was moved to this server?

Or you have had some AD domain changes / consolidation?

If the security was added under a different domain, the Group that you see is not the SID you think it is.

This would be similar to moving a db between servers, except there local logins are being affected.

If you can easily remove, then reapply the security, this would be an easy test.

Just be careful if you have other security applied based on this group.