I'm not sure the exact timing of updates on my 2017 Shield TV for either the Channels DVR app or the system software, which was fairly recently updated to the latest Nvidia version, but I started to notice some pixellation on one of my channels, the local CBS affiliate, which is an over-the-air 1080 station on my Quatro tuner. The signal levels for this station are very strong, and had never had any issues until recently. Last night, I switched the player setting in the app to "software" rather than "hardware", and this seems to have solved the problem. I'm not sure that the software decode setting will have any other negative impact to playback from other stations. I did not see this same issue on any stations tuned from my Cablecard prime, nor on other broadcast channels.
Some OTA station encoders use encoding techniques which trigger a bug in the shield's hardware decoder. Nvidia is aware of this issue and has stated this is a hardware level issue that they cannot fix. That's why we added the software decoder option to the app.