Hello

I have the same problem. He's been fighting with him for a long time. I tested Explorer 50 and Voyager 5200, but it's the same. With Voyager, the volume of the music is slightly reduced every several seconds.
I tried with the PLT application and without it. I tried to turn off the A2DP and it did nothing.
Of course, turning off even the entire location function has no effect.
I connected with BT SONY headphones from the NWZ series and did not interrupt the connection there. Is it possible that the problem lies on the Plantronics headset side if it occurs even when their application is not installed on the smartphone?

Has anyone got a way?

Yes, after latest update, v1.0_20190320 the bt is working with my plantronics e50. Also you can find that MTK Engineer mode is back (Thanks dev team!). Also it seems that apps runing in background do really run in background (updated phone 1 day ago so no extensive tested). Another good thing: MiraVision is back! You can get rid of that cool white light of the screen in the evenings.

The last update of the software solved the problem.

Blackview is fixing all the issues, just give us some time to do it