I have a similar problem with Youtube - whereas yesterday I could see several thumbnails in at least 2 rows, now I can see literally 2 (and the tops of 3 on the next row).
I assume then that this isn't a YT app issue but one with the underlying UI features. I did think the player had decided it was 4k and thus showing double sized (even though my roku ultra doesn't do 4k, its set to 1080p) or that the app had broken, but if you're having the same issues then its probably Roku at fault.