Just to keep
Dazzler happy (it is important you know
) and as a temp workaround - you could do as he suggested... USB stick for music.
Unfortunately I don't have a Samsung Galaxy S6 (or S5 - per the other thread...
Samsung Galaxy S5 phone issues) - nor UK navigation headunit (or a GD i30 for that matter). However I did a little research - seems likes lots of people with S5/S6, etc. are having a variety of Bluetooth issues with a variety of cars and/or headunits! So it isn't just you. Seems many of them didn't have the issue with either a) an older phone; and/or b) older Android version. Many of them seemed to be having the case of few years old car - new phone (or updated to latest OS version) - apparently clearing the phones Bluetooth cache was a fix for OS update issues (don't think so in this case).
Two things seem to be semi-common as possible fixes....
Smart Lock (google app - so guessing part of Android) apparently causes Bluetooth issues - with some people saying turning it off fixes issues - I would give this one a shot first.... Here is a Czech guy turning it off:
Android lollipop problem with Bluetooth connection - YouTube and describing it as 'Smart Lock this is a bullshit' - note he restarts the phone after turning Smart Lock off, then erase all (bluetooth) data and restarts again. I think the erase part is the same as detailed here under 'How to fix Samsung galaxy S5 Bluetooth issues.' here:
How To Fix Samsung Galaxy S5 Bluetooth Issue ? TechnoBezzI am not sure how it would fit into your situation, the fact the hands-free works but the track controls don't - but maybe the track controls are seen differently to the hands-free controls?!?
Another thing that seems to be causing issues is MirrorLink (implemented in Android 5) - some people also seem to have issues having it off an stay off. This seems to be more related to the audio/video site of things (rather than phone)... But, as far as I know, it needs USB connection - so don't know why it seems to be Bluetooth related (or maybe people are using BT and a USB cable and confusing what is happening) who knows...
At the least I would suggest it is something to check and/or turning it off - seems to be in 'Settings -> Connections/More connection settings' this page shows turning it on:
How do I use MirrorLink ® to connect my Samsung Galaxy S6 to my car?
One other thing that could be worth a shot is taking a screenshot of the details of the BT connection to the head unit and posting it here (see elsewhere for imgur/photobucket use) - in case someone can spot something 'odd' in them (sought of doubt anything - but who knows). See the following on how to get to details
Connect to Bluetooth - Nexus Help and here on screenshot
How to take a screenshot on the Samsung Galaxy S6 | Android Central (apparently you can also swipe with your palm left or right as well)
FYI here is an article on how to turn on SmartLock:
How to: Enable Lollipop's Smart Lock in TouchWiz on the Galaxy S5 | Droid Life