I have a Joying stereo and had exactly the same problem with v3. After reverting to v2 the functionality was restored.
It was a little more challenging for me, cuz I replaced my coreboard and went from Android 7 to Android 8 at the same time with upgrading to v3. So, I suspected something is wrong with the new firmware on my unit and was about to write to manufacturer about the problem. But after finding this thread and downgrading back to v2, the steering wheel buttons functionality is restored.
I hope dev will investigate and fix this issue, as it's affecting a lot of people which are using Poweramp on their Android stereos and purchased Poweramp to support dev.