I have a new Asus Zenfone 3 I bought back in February. I'm having issues getting my bluetooth devices to automatically start playing when paired.
With all my previous phones, I can pair the device with bluetooth speakers (i.e. car stereo or portable speaker), press the Play button on the bluetooth device, and the phone would start playing music from the last app. This worked with a Samsung S2, LG G2, OnePlus One, and OnePlus Two. The process should be the same, in my experience.
On my Asus phone, I have tried with both Vanilla Music Player (F-Droid App Store) and the Google Play Music app. I have started songs, got to my car, and have it continue playing. If it stops for whatever reason (i.e. a phone call), then I have to fiddle with the phone to get music playing again.
My goal is to have my old behaviour without being forced to wipe and root it - connect to a bluetooth speaker, and use the bluetooth's controls for the music app even if it's not currently running.
The phone is running Android 7.0, phone model number is ASUS_Z012DC