The following HTML:
<img data-thumb="//i.ytimg.com/vi_webp/BQOXkEU-0T0/mqdefault.webp" src="//i.ytimg.com/vi_webp/BQOXkEU-0T0/mqdefault.webp" alt="" aria-hidden="true" width="196" data-group-key="thumb-group-0">
Turns into this when I load it with the jQuery load function:
<img aria-hidden="true" alt="" data-thumb="//i.ytimg.com/vi_webp/BQOXkEU-0T0/mqdefault.webp" src="https://s.ytimg.com/yts/img/pixel-vfl3z5WfW.gif" width="196">
The main problem is that it turns the src to the webp file into a gif. The src turning into a gif makes the the thumbnail unreachable because it doesn't exist.
Update: I just realised that there is a good chance this is on youtube's side. Maybe they automatically send gifs instead of webps to certain requests.
Update: I still have no clue