Bluetooth signal strength bug with my car's stereo
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.