I think I’ve figured out part of this.
Windows users could still write to the volume but they saw 0 bytes free as well.
What I think was going on was that I was serving the smb volume from the head node on my cluster which was auto-mounting the user directories via NFS from our storage server. Thus the directory that had all the home directories in it didn’t really have any space and the actual directories under it didn’t get mounted until you went into them.
When I log in to a specific user’s directory it auto mounts it on the head node and samba reports something more reasonable. (My PowerBook still shows 200GB when it’s really 1.9TB free.)