But the original DASH implementation was very aggressive about keeping a minimal buffer size and downgrading quality at the slightest hiccup. I was also on a not particularly great internet connection at the time, so I used to pause 720p at the start and let it buffer enough for uninterrupted playback, but YouTube's changes meant that just buffered like 15s and then shifted down to 360p. There were add-ons to force dash off to mitigate the problem but iirc YouTube was starting to make higher qualities dash exclusive.
Clearly someone at YouTube was optimising to reduce time-to-play and buffering but in a typical metrics driven approach, excluded the possibility that for some videos these were less bad problems than 360p videos. At that time I was watching a lot of let's plays and programming tutorials and most of the people were uploading content recorded and intended for 720p or 1080p, so unreadable text because of quality downgrades was a big problem for me at that time.
Nowadays I just brute force it by having comically excessive amounts of internet (2gbps ftth) so YouTube randomly deciding garbage quality is much rarer, but it still happens sometimes.