Update:
I deleted the files off the SD card's music directory.
I installed the latest MM beta 5.0.0.2333.
All selected playlists are syncing now.

I've had all kinds of playlist syncing problems but this is new:
Only 30 of 60 playlists sync.
Connected directly to computer via USB3.0 (SD card).
Appears as a drive letter in Windows 10.
MM5 Portable Mode v5.0.0.2326

I'll try to capture a debug log but that ends up causing a crash in itself since dbgview runs into 32bit memory limitations. Do you have another way to diagnose this? I can let you remote control and you can see for yourself.