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)
Same problem here and it has not been addressed as of 3/21/21. The older three Roku units I have do not have this problem; only the Ultra 2020 unit.
Since there hasnt been a firmware update since early February (9.4.1-4236), and thats the only one since the OP (not that the issue couldnt have or didnt predate the OP), you'll probably have to wait another build or 2 before this is likely to be fixed (6-12 months or never are not uncommon fix times for Roku).
Its also likely OS 9.5/OS10 will drop before another 9.4.1 build does at this point (should be announced at the end of March/early April), and that will likely add to the fix time.
Thanks for the response. I guess we'll have to wait. I do have a workaround by using my Nvidia Shield TV Pro for the SD stuff.
-Jack