Having an issue that I can reproduce very easily. Started experiencing this around 6 weeks ago.
Galaxy S7, syncing to a 128GB external SD card. The SD card is encrypted, but like I said, this has never been an issue before. When I reset my phone, then open Plex, the sync location is automatically changed to internal. In order to play/sync anything, I have to switch back to the external SD card which brings up a prompt that warns you your synced content is going to be deleted. Click 'OK' and it switches back to the external SD, and deletes all synced content.
Currently resyncing 30GB of media (again). Looking for a solution or is this a bug?