Comment by judge2020

I remember them starting to do automatic lower-quality streams when this came out[0], but I'm not sure if this is still the cause for the situation. It could be a general "we see this ISP/ASN failing more often with x many concurrent 4k streams, let's throw some people on 720p and see if it helps".

0: https://www.pcworld.com/article/398929/youtube-defaults-to-l...