Recently I’ve made another observation, which may help Jolla to trace the root of the issue:
Normally I have to restart Bluetooth subsystem through Settings - Utilities.
If I press the button Restart and immediately switch off the screen, Bluetooth does not get activated. So I have to keep the phone / the screen active, not sure if it’s even necessary to have settings in foreground.
I’m wondering if foreground or power save behaviour may interfere with initialisation of Bluetooth in the boot process.