The problem. 480 p & I mpeg2 mkv files which are anamorphic/widescreen, but playback is at a 480 width. The effect is that the picture is squeezed and out of proportion.
I run both Plex and Emby. Both player clients, and many files, all exhibit the problem behavior. Because of this, I believe this is a Roku software issue.
Try testing out RMP as well.
Regardless, you need to contact Roku directly about this:
chat or email: https://support.roku.com/contactus/contact-options (choose setup/ultra/other/need more help?)
and/or contact @RokuDanny-R and/or @RokuChris (Roku Media Player)