I believe it's expected behavior. If a backup fails, you get a report of that on the client being backed up, and on other clients if they're set to receive alerts. If you choose to leave that backup, well, it
is the first backup of that client for that month. Even if it only finished backing up one drive of 5.
You can still submit a bug report or product suggestion...
I'm not on the WHS team, I just post a lot. :)