In desktop player, cover art strongly prefers a traditional, tall rectangle Hollywood poster dimension for cover art. However, the Roku Plex app forces cover art into a horizontal cutout, and crops rather than resizes the artwork.
Also, the Home screen automatic sections screw up custom covers, completely ignoring them.
I have the same issue (and have reported it previously to no avail). This problem has been present for years and has never been addressed.
Compared to the other device clients, the Roku client is inconsistent with regards to displaying cover art for “other videos” libraries. All other clients use the “poster” metadata, and display it in a tall, portrait manner. The Roku client takes the erroneous approach, and takes the “poster” metadata and crops it into a wide, landscape manner, which is completely unacceptable.