Bluetooth signal strength bug with my car's stereo i have problem with xiaomi mi a1 and bluetooth radio of ford fiesta car. 

I connected an Android phone to a vehicle via bluetooth to make/receive calls.

Connecting the device via bluetooth was simple. Making calls, however, was a challenge. The vehicle reports that the Android phone is always without cell reception, and therefore will not place calls. Furthermore, the vehicle's head unit (aka receiver) shows a "no reception" indicator for the Android device's cell reception.

Placing calls from the Android device through the vehicle's audio system works as expected. This also shows that the Android device has ample cell reception to make calls.

with the bt hci log i see that:

when phone connects bt to radio, the phone sends this info:

+CIND: 0,0,1,3,0,4,0

legend +CIND:("call",(0,1)),("callsetup",(0-3)),("service",(0,1)),("signal",(0-5)),("roam",(0,1)),("battchg",(0-5)),("callheld",(0-2))

when the signal strength on the radio disappears and reset all (zero) the phone send this info:

+CIEV: 3,0 +CIEV: 4,0

3 = service, 0 = no service (previously was 1) 4 = signal, 0 = no signal (previously was 3)

but the phone in realty upgrade the signal strength from 3 to 4!!!!!
At first sight the source of the problem in your log is mIsSimStateLoaded=false.
The true value of the service indicator (1, available) is logged, but because of this SIM state equal to false HeadsetService sends 0 (not available) instead of 1.
The true value of the signal indicator is unknown, because of service=0 HeadsetService sends 0 instead of the true signal level indicator (this temporary 0 value is logged, in fact it is not mSignal value).
Possibly Android HeadsetService contains an error with mIsSimStateLoaded flag processing, because it can be cleared on every phone service loss, but set only on ICC_LOADED broadcast. If so it is strange because a lot of Android phones shoud have the similar problem.
Have you tried to reboot the smartphone before connecting to headunit?
Upd: Hmm, this SIM broadcast is sticky, than maybe this is bug by Xiaomi, not Android.