You could try reinstalling the YTTV app, but it wont impact this issue at all.
if the Auto (default) setting for 720p/1080p/4K channels results in macroblocking/low resolution, the solution is always to manually select 720p/1080p/4K (once for each resolution) - this is the recommended configuration regardless.
"Auto" relies on stepping/ramping from very low resolutions to begin coupled with network conditions to determine the resolution.
However, even with manual resolution selection, this may occasionally need to be re-done, due to app updates, firmware updates, changed network configuration, or changed network conditions (e.g. dropped packets/low bandwidth).
Make sure your network conditions are reliable (e.g. if using WiFi, try using 5Ghz instead of 2.4Ghz, or switch channels) to prevent/minimize this from occurring, and be prepared to occasionally manually configure each resolution.
