I just tested AP mode, and the router doesn't assign any IP to the eth0 interface, so it's not that the mode check was backward. I suspect it was a mistake as they removed the check for router mode when they also removed that nvram var that allows you to force WAN binding.
I'll put back the former mode check so the router won't bind Samba to the WAN interface again while in router or AP mode.
If you need an immediate fix, a temporary workaround would be to copy your smb.conf to /jffs/configs/ and then edit the "interfaces =" parameter to remove eth0 from it. After restarting Samba, it will start using that custom config file. That means you will have to re-create that custom config if you ever do any change to your plugged USB shares.
I'll put back the former mode check so the router won't bind Samba to the WAN interface again while in router or AP mode.
If you need an immediate fix, a temporary workaround would be to copy your smb.conf to /jffs/configs/ and then edit the "interfaces =" parameter to remove eth0 from it. After restarting Samba, it will start using that custom config file. That means you will have to re-create that custom config if you ever do any change to your plugged USB shares.