Some time ago I became aware of an issue that caused VLC for Android to limit the number of items it showed in a single network directory, over SMB.
There were 160 tracks in the folder and VLC only showed 106. It turns out that track 107 had several emoticons in the filename, such as a headphone and music notes. After removing the emoticons VLC shows all 160 tracks in the folder.
Right before the issue started I had trouble with the drive on which these files are. I had to take it out of its original USB 3.0 enclosure and put in an older one. While the drive became accessible again, I started to notice the issue described above.
I do not know if the issue of not all the tracks being displayed was occurring before I swapped the drive to the older enclosure, but I am pretty sure it started after.