At my phone, bluetooth is behaving as normal after enabling flight mode, if there has been no bluetooth device connected to it during enabling the flight mode.
if a device is connected duri g enabling the flight mode, I cannot turn BT on again after enabling the flight mode and a restart of the phone is needed.
So my workaround is:
Switch of my BT headphones, than switch on flight mode, switch on BT again and switch on the headphones again. No problems with this sequence so far.
But of course it is a workaround and should not be an intended behaviour 