After doing some searching, apparently AVRCP 1.5 was introduced back in March with build 14283. A few days ago I installed build 14342.1004 on my Lumia 950 which was the latest fast ring build before today but couldn't get it to work. This used to work fine in my car on my Lumia 830 with Windows Phone 8.1. You'll see AVRCP 1.5 referenced in build 14283 here under known issues: