I have Windows 10 and VLC media player 3.0.17.4 with, I think, default settings.
When I convert an mkv file to mp4 the result plays in VLC but not in Windows Media Player. The error that I get is "Windows Media Player cannot play the file. The Player might not support the file type or might not support the codec that was used to compress the file."
The files that I had converted to mp4 with an older version of VLC (and some non-default settings, but I don't remember which ones I had changed) still play ok. The issue came up after I updated VLC.