Just tested a fresh install of SW 2.51 in parallel with an existing SW 6.0b on a Win 10 LTSC workstation. All appears to work properly for video and audio with latest codecs installed. For reference, MKV files were used for evaluation.
Some suggestions:
Use a program such as gSpot
https://www.videohelp.com/software/GSpot
This will identify currently installed codecs and also properties of your media for comparison.
Note: In the past, I recall having to install splitters such as Haali, but I believe now that LAV is installed with K-Lite. I’m not sure, but if both are installed this might create some problems.