It takes around a minute or so for my iphone to acknowledge the connection lost to the f series bluetooth. The whole bluetooth voice/mic exchange seems to be quite buggy, I have never been on a phone call though when i tried to pas sit back to the headset, are you saying it wont even do it manually?
I have a feeling this is more due to the bluetooth connection not being terminated yet then anything else. It's just a time out, rather then the unti saying, shut down the connection im turning off now.
I don't think there is a fix for this, its more or less something you have to switch it tot he head set manually just before you shut the car off. That is probably the easiest way to deal with this.