I updated to 1809 today and can confirm that the problem is fixed in this release. I think this thread should be closed.
So glad it is fixed for you, but I updated to 1809 and my Bluetooth device media controls function exactly as before, which is to say the Volume buttons adjust the universal Windows 10 volume, and the Play/Pause /<<</>>> keys do nothing at all in any media player I tried, including WMP.
CORRECTION: The Play/Pause button activates Cortana in 1809 if I press it just right.
I've uninstalled Intel Bluetooth, deleting the drivers and let them reinstall & removed and re-added the devices. Nope. Same non-functionality as 1803.
It's a huge disappointment as I've been waiting (im)patiently since April to get this non-trivial functionality back. :(