IIRC there have been similar problems reported here with the Galaxy S5 after upgrading. I think it started with the 2013 version (which is the same BT files as 2015). So maybe there was some incompatibility between the S5 software and last Pioneer (Parrot) BT. You might look for those threads. I think some people fell back to the older BT software - I forgot what version. If you can find the older files you can install them the same way through the BT settings menu.
I was having some BT audio connection problems after the 2013 update. I have a Nexus 5. But it cleared up after newer releases of Android. I'm on v6.xx now and my only BT problem is re-connecting to a BT source when I start the car. It seems to timeout waiting for the phone connection. It doesn't happen every time. I just need to wait for the phone to reconnect then toggle to a non-BY source and back.
Another option (if you feel comfortable hacking your phone) is to install a custom Android ROM on your phone. The xda site (link) has several S5 ported Android v6.xx ROMs with many customizations and features. Now guaranty but that may solve your problem.
So..How about a way easier method. Pioneer on the phone told me (a blatant lie) that it was impossible to revert the bt fw back to 3.24.
I just found a post on here titled "z140bh Bluetooth audio problem". There's a link to download a 2.3 gig file containing the 2012 software including the the bt fw 3.24. It worked I just did it and now i am so relieved that i don't have to go out and spend $1k plus for a new HU. Not having something work drives me insane. I guess I'm ocd that way. I have the need to fix things and make things work lol...That post has great instructions. Super easy. Get this info out there...and the name of the file is cnsd-310fm
My pandora is controlled from the HU and plays through my truck speakers again.