Just as a follow-up shopgirl284 - When you mentioned that it was an .mkv file I decided to check a couple of .mp4 files that I have. The three files I tested all played fine. I tried to play 3 .mkv files and they all stopped playing within a minute or two.
I’m not sure if that helps you narrow down any issues but thought I would let you know. But again - all files (mkv and mp4) were working fine on 9/25 and the mkv files stopped playing on 9/26 when my app was updated.
The only other thing I can suggest trying would be increasing the video quality settings in your Plex App for Roku
under Settings > Video > Local Quality (Local Quality - media on a local PMS, Remote Quality - media shared from another user’s PMS, and Online Quality - online media accessed through channel plugins and Watch Later). I know that when this is set too low, it can cause media that should direct play to transcode. So it is worth a try.
Otherwise, someone with more specific knowledge of the Roku player software/SDK and how the player programming of the Plex app for Roku player may be different from the player programming of the Roku Media Player channel will have to review your issue and the documentation you have provided in this post to see if they can determine the exact cause.
I am just a volunteer. So I can help to make sure you have ruled out the basics and help you isolate and document the issue, but at this point someone more familiar with the specifics of the Plex player app programming will have to review this. And thank you for including the specifics detail and documentation about the issue you are having and your troubleshooting progress. That should help to isolate the issue and cause.