That's an unusual symptom. So I think you want advice from some of the other experts. As I said before, you're in the wrong forum to get that.

Your remote supports learned signals and keymoves and macros. Any one of those might be responsible for your strange symptoms.

I'm sure the HT mode cannot choose a different device for Vol+ than Vol-, but from your description I'm not sure which device is set as the HT vol device.

I also don't remember whether your model remote supports learned signals and/or KeyMoves specific to HT mode, or whether those must be in the device mode that is programmed for HT vol.