I'm having this same problem. Most of the playlists on MediaMonkey for Android are complete, showing only about half the songs on the playlist that are present on MediaMonkey in Windows 10. I re-synced one playlist that had only about half the songs on Android. For a moment, all the songs in the playlist appeared, then the screen refreshed and around half of the tracks disappeared.

MediaMonkey 5.00.2338 (Windows 10.0.19042.964)
MediaMonkey 1.4.3.0950 (Android 11, Galaxy S21 Ultra)

I am trying to sync from my media monkey for windows version 4.1.28.1905 to media monkey for android version 1.3.9.0924. The playlist is missing songs, and occasionally a playlist I am doing nothing with that had previously synced properly loses songs for no apparent reason. Sometimes it loads a playlist with no content at all. I have tried syncing wirelessly, I have tried syncing with a USB cable, I have tried syncing through the tools drop down menu, I have tried syncing by right clicking on the playlist and sending it to my phone, I have tried deleting the old list and resyncing, I have tried just leaving the incomplete list and syncing again. Sometimes it adds a few songs, sometimes it takes some away, occasionally it gets all of them there but then I sync a different playlist and the one that was already there loses a few songs and the one I was actually syncing is missing a bunch. I have deleted the app entirely and tried to start from scratch. I have scrolled through this section and tried virtually everything else suggested but the only thing that seems consistent is that syncing MM is a pain in the ass and if it doesn't work properly once, then it never will again. Is there actually a solution to this?