I think I have a permanent fix, without crazy reflashing.
1. Settings
2. Apps
3. Tap 3 dots at the top, "show system"
4. Scroll to Bluetooth Share
5. Tap Storage
6. Clear Data, then Clear Cache
7. Back to Apps list
8. Tap settings cog icon
9. Battery Optimisation
10. Show all apps if not already shown
11. Scroll to Bluetooth Share
12. Change to Don't Optimise
13. Reboot phone
Didn't work for me. What a waste of money this phone has proved to be. Charging cable area that doesn't allow a standard charging cable tip length and now this unstable bluetooth and i'm only in week one
For the bluetooth, that was driving me crazy until I've done this : http://community.umidigi.com/thread-15973-1-2.html . That's not easy, but not impossible too. Almost 4 months later, I can confirm that bluetooth is stable.
But yeah, I will definitely not buy Umidigi's phone anymore.