I have similar issue w/ iPhone X / 2017 Ford with Sync 3. This thread identified the cause.
Mine definitely started after iOS 13. Never did it before. (Therefore it worries me that posts at beginning of this thread are from before iOS 13.
Have tested mine, and it will not do it if just start car, allow Phone to connect, even if let idle 5 minutes, then shut off car.
I have to actually drive car for at least about 10 minutes. And then, when I stop, shut down car, I find the volume is way down from the 90% it started with. Have today tested on 5 short drives...did it every time. For me, volume does not go to 0, it goes to about 10-15%, but still too low to hear ringer.
Have missed many phone calls due to this.
Have not tested the workaround of the side button ringer adjust setting. Guess going to be forced to try that, as can’t adjust ringer back up ever time get out of car.
But don’t like, I have long used the buttons to adjust ringer volume (and to double check where the ringer volume is set).