Why Android-Auto is setting the loudness of Ringtone on my smartphone to ZERO, when I disconnect?

[AC Question]

When I leave my car (Opel CORSA with Radio Intellilink 4), Android Auto is setting the loudness of ringtones on my smartphone to 0%. So I have to reset the loudness every time after I have disconnected my car and my smartphone ( Moto G4+; Android 7)

Regards
Manfred