Welcome to the forums,
Syncing directly to my Samsung Galaxy S5 has never worked for me here or on Mediamonkey on either Android Marshmallow or Lollypop versions because the file transfer protocol Android uses (MTP) just plain sucks. Well, syncing my phone sometimes works on Mediamonkey as well as sometimes trashing my entire collection on my microsd. Now, all I do is to take the microsd out of my phone, place it into its caddy and slot it into my computer's sdcard slot. With this setup Musicbee syncs my music reliably and quickly, whereas Mediamonkey seems to have the same habit of trashing my collection now and then. Yeah it sucks that we have to go through this, but whenever I pine for the iPhone I quickly remind myself the mess that is iTunes. Hope this helps.