Bluetooth loses connection to car kit speaker while still muting radio

REPRODUCIBILITY (% or how often): 100%
BUILD ID = OS VERSION (Settings > About product): 4.1.0.24
HARDWARE (XA2, Xperia 10…): XA2
UI LANGUAGE: German
REGRESSION: (compared to previous public release: Yes, No, ?): Yes

DESCRIPTION:

I use my XA2 in my Mercedes model year 2005. Until the last update, the bluetooth connection to the car kit was reliable.
Since the update, the phone loses connection to the speakers - it just won’t play the sound any more and on the phone app screen the button for the headset disappears. This happens all the time - even while having a conversation. The workaround is to delete the connection and reconnect bluetooth from scratch. After a new connection, I can make new calls again, but after some time (days), the connection to the speakers is being lost again.

PRECONDITIONS:

Bluetooth connection.

STEPS TO REPRODUCE:

  1. Connect bluetooth car kit
  2. make calls

EXPECTED RESULT:

The connection will work without problems

ACTUAL RESULT:

The connection to speakers gets lost, while the bluetooth connection is still active and mutes radio.

ADDITIONAL INFORMATION:

(Please ALWAYS attach relevant data such as logs, screenshots, etc…)