Related anecdote:

Just a couple days ago I was watching YouTube and the videos kept defaulting to 480p quality and not automatically increasing to a higher one. This was on my home network that is definitely able to handle a higher quality stream. I was able to manually set it to 720p (good enough for watching on my laptop) and it didn't flip back. This happened a few times in a row that I found it odd. I then thought it might be ad blocking related so I disabled uBlock Origin for YouTube and then everything worked fine after that. Coincidence? Maybe, I need to try this a few more times to see.