Another A12 type II back came into my possession with same problem, when starting a new roll, the counter didn't stop at 1. I pulled up the leatherette around the crank. Took out the 3 machine screws that hold the cover on. This exposes the clockwork of gears and levers that are so beautiful. I blew out the gears with some gentle compressed air, but it was clean. I used a bit of my movie film cleaner to no effect. The mechanism was dry. I found the pawl that clicks into a notch near the number 7 on the counter wheel. To the best of my knowledge this is what stops the counter at 1. I used Rem-Oil, lightweight machine oil and lubricated this little lever. This seems to have fixed it. I wouldn't be afraid to remove the cover, but actually repairing, removing parts isn't my cup of tea.
This makes 2 backs that I've got working with a bit of solvent and a couple drops of very light oil. These backs are from the 80's. But newer versions are hard to find that aren't nearly 20 years old.
This is the beauty of the Hasselblad system, you need to maintain them, but there's no integrated circuit, etc to completely kill the beast. Also if these, like a watch, require maintenance, there will be people who can make a living maintaining them.