Are you on T-Mobile? If so, turn off Binge-On and see if that helps.
Does this happen when you're on wi-fi as well?
Yes I agree with you very annoying. Every single video I watch I have to manually change it.Having strong or good signal is no indication of your throughput of the system. If your on a congested tower or during peak times you will get full signal but may have crappy speeds.
I have yet to find a way for my youtube app to keep the higher resolution settings. Seems to change on its own. Rather annoying.