Player Version#: 7.21.11…
(on Roku)
To reproduce this bug, browse your list of Collections and select one with custom poster art that was cropped (automatically and satisfactorily imo) in order to fit within the limits for Plex media.
When viewing that Collection’s detailed page, the poster art is not (noticeably) cropped and is instead squeezed horizontally (reduced in width) to an obvious degree.
I don’t see this behavior in other player apps such as the Mac, web, or AppleTV clients.