Today I installed a Comcast DTA and had exactly the same problem with my Magnavox 15MF605T. So I went looking for solutions -- which got me here.

Codes 10037 and 10054 will turn the TV on and off, but the volume only goes one way (up), regardless of which volume button on the remote is pushed. I also found that the Mute button turns the TV on and off. But, if you turn the TV off with the Mute button, you can't turn it on with the Power button; you must use the Mute button again. I have two other remotes that work properly with other TVs, but they do exactly the same thing to the Magnavox 15MF605T.

The programming in the Comcast remote must be at fault.