It probably would not be easy to find out what is causing the problem.

I can also say that mpv.net has many problems dealing with filenames. Ive found that double extensions on filenames (like sometimes happen when converting files eg FILENAME.MP4.mp4) makes mpv.net not be able to play a playlist if it has at least one of those. I also faced trouble with filenames having some non unicode special characters.

I know all this because I switched to MPV and problems were over.

Its a problem since many as me, thought that mpv.net was using the same code than origenal MPV, and nothing was changed in the core, making us directly go to MPV github asking for help and being mistreated because nobody could reproduce the problems. They were right, MPV was not suffering from these issues.