I had the same experience. The available codes would operate POWER, VOL + only, MUTE would do power, and there was no VOL -. Sometimes I would see volume drop one notch before going back up.

I wrote to Comcast customer support and even gave them the link to this thread. They gave me a boilerplate reply suggesting going to their office and swapping out a defective remote. I was about to write back and say what is the point because unless the vendor fixed crappy firmware it will do no good to try a different remote.

BUT...

I decided to try the other remote (we have two of the mini cable boxes). After resetting the first one for the other TV and testing it (that one is a Sony), I took the one that had previously been with the Sony, set it with 10054 and tried it with the Magnavox. Much to my surprise, it works ok. Mute is fine. VOL - is fine.

So it's possible that there are remotes out there with corrected firmware. The two digital adapters we have came as part of a shipment with a full cable box when cable boxes became mandatory so one might assume they were part of the same batch but apparently not.

So if you are having this issue with this model go ahead and head up to Comcast and try a different remote. You could even take the TV along; it's small enough. Print this out so they see it varies among remotes and let them find you one that works. (It would also embarrass them if they can't get it to work.)