Finder reverts to SMB for shares set up as AFP
Apple is switching to SMB, as multiple sources reported. And you can see this from the Apple Support article:


  When you connect from a Mac using OS X Mavericks or OS X Yosemite to another computer using file sharing, your Mac automatically tries to use the Service Message Block (SMB) protocol to communicate. If SMB is not available, it tries to connect using Apple File Protocol (AFP).  (How to connect with File Sharing on your Mac - Apple Support)


In fact, I have to disable SMB protocol on my NAS to use AFP. I think I will continue to do this till SMB supports case sensitive device names.