AD Domain Security Groups

[DCARichM]

Model: TS-EC1279U-RP (x2)
Version: 4.3.4.0644 & 4.3.4.0551

I'm trying to add an AD security group to a share but the NAS cannot find the group I want. It can find other groups in the same AD container. I've tried this on both our QNAPs and they're both having the same problem.
The security group in question is a Windows built-in one; Backup Operators. To get around this for the time-being, I've added the service account I need to the share but I'd like to know why the NAS cannot find the group. There are other groups in the 'Builtin' AD container which cannot be found: Replicator, Terminal Server Licence Server, etc and some groups it can find: Access Control Assistance Operators, Hyper-V Administrators etc. Comparing the groups side-by-side, there is no real difference (apart from the names, etc.).

Has anyone come across this problem or anything similar?

Thanks in advance.