Easiest solution is just to use the highest resolution thumbnail the official documentation says is supported on all videos (vs "some"), which is hqdefault.

https://developers.google.com/youtube/v3/docs/thumbnails

Maxresdefault is overkill for this purpose IMO. Why waste the data? Incidentally this is why I run an add-on that redirects all youtube thumbnails to mqdefault, it saves me a nice chunk of bandwidth / memory / perf.