some .mkv files are black screens with audio working okay

[bleepbloopplop]

I uninstalled, re-installed VLC ... did not resolve the issue.
Searched the forum and did not find a thread with this specific issue.

macOS 10.15.1 (Catalina, stable version)
Only 1 monitor - the iMac itself.
No macOS updates available.

VLC 3.0.8 Vetinari

I've never had a problem with VLC on this iMac before. For whatever reason now, I have .mkv files that play audio but the video screen is black.
Is there a recommended set of codecs that need to be installed along with VLC now?
I'm not sure how to create logs for VLC, but I'd be happy to do so if it will help figure out the cause. Instructions?

Thank you.