You can refer to this workaround, even if it was made for the BV8000 Pro:viewtopic.php?f=191&t=3331900) Consider updating your device to the latest firmware and performing a factory reset after that
1) Go to "Settings" -> "Background task clear" and whitelist all your 3rd party apps wich need bluetooth to transfer data
(i.e. samsung gear app, veryfit app and so on..). By doing that, they will be allowed to run in background even if the smartphone is in deep sleep state.
2) Go to "Settings" -> "Battery" -> "3 dots" in the upper right corner -> "Battery optimization" -> select "All apps" from the dropdown menu and disable battery optimization for every bluetooth service app, as
- Bluetooth MIDI service
- BT Tool
- Bluetooth sharing
and for every app needing to use bluetooth to transmit data, as
- Phone services
- Call management
- Phone/Messaging storage
- your 3rd party music player
- other telephone dialer apps
- fitness band apps and so on
3) Only if the previous steps doesn't solve your issue, ALSO:
- Go to "Settings" -> "Battery" and disable "Standby intelligent power saving"
- Go to "Settings" -> "Duraspeed" and and disable it.
4) Only if, again, the previous steps still don't solve your issue, ALSO
go to "Settings" -> "Battery" and set your device into "High performance mode"
Please follow these steps carefully and report back if your bluetooth issue gets solved ;)