Just got the Marshmallow update this morning for my phone, and now I lost almost all of my Amazon offline playlists. One playlist came through intact, but the others all show either zero songs or a few songs where there should be like 20 or 30. I'll take part of the blame for not fully synching to my Amazon online library, however many of the songs are in wmv from back when I converted my CDs to digital and Amazon doesn't let you put those on the cloud, and many just followed me on the SD card through several phones.
I think what happened is that Marshmallow changed default storage location pointers. The S5 phone has that funky "internal" SD-like memory, separate from the SD card. Of course I'm leaving to go fishing tomorrow and wanted to listen to my offline music on the road. Now I have to spend a couple of hours remembering what songs I had on what playlists, and putting them all back together. Rats.