that is a 'feature' of recording a series by 'search'. If you use search to find the next episode (date/time/channel - for example in HD-) you can go to the tv listings and hit ok or red record button and add the series there. It will present the option to reocrd that channel only'.
also, might want to look in "press xfinity button >> gear(settings) >> preferences >> General and select auto tune to HD channels to toggle to ON.