Why don't groups work?

I have a readyNAS 102. I have two local users which correspond to the Windows (local) logins on two machines. Those user accounts on the NAS belong to a group called "users". It seems after the most recent firmware update to 6.4.0, shares that are read/write for group "users" are no longer accessible to those users, unless I go into SMB security and explicitly check the box for "read/write" for each user. It's like the group access policy is completely ignored, and those users are asked for credentials (which fail every time for some reason).

Is this a change in 6.4.0 that anyone else is seeing? I haven't changed anything about my shares; they've been there along with the same users/group since I first installed this device on the network, and just recently I've been unable to access shares unless I grant each user explicit permission.

It seems to defeat the purpose of having groups, unless I'm missing something about the way groups are supposed to work. It's fine since I only have a couple of local users, but it seems like something is wrong.