As far as I know, Flyme OS, like MIUI OS (from Xiaomi) are using severe or aggressive background killing processes policies, in order to free up the available memory. Maybe this thing has some influence on BT disconnecting, maybe some background processes that are needed for BT to work correctly are wiped from memory, leading to BT connection errors. Of course I have no proof for this, but it could be a reason.

This, or firmware inconsistencies regarding BT functionality are faulty.