It does that for me too, I have gigabit. The thing is, YouTube is using an algorithm and it's pretty dumb, you have to keep forcing higher resolutions and it will learn eventually. But sometimes it will still make it lower. It's doing this by design, if you surf recommendations and go from video to video it will try to sneak in lower quality to save YouTube money.
If I go to random channels I rarely visit it doesn't even give me 720p, it tries to give me 480p.
Edit: Oh and I have YouTube Premium too, doesn't help.