I don't quite follow. Wouldn't you want to use your car's speakers to hear the navigation directions, as they would be much easier to hear than a phone speaker over road noise, etc. Am I missing something?
My complaint about Maps is I just discovered they removed the option to independently adjust the volume of navigation instructions, so now they're absolutely blasting loud if the music that I'm listening to has to be turned up to a certain level. I really don't understand what developers are thinking sometimes. Why the heck remove a completely good feature for no apparent reason? Same with your issue (even though I don't quite understand the usefulness of it . . . help me understand).
That is the thing, I don't want the navi direction thru car's speakers. in iOS14 there was an audio output menu for navigation voice which Apple remove now
this is what ios14 user manual says about it
“Change the audio device for voice directions
With turn-by-turn directions showing, tap the route card at the bottom of the screen, tap Audio, then choose a device (below Output).
Devices, such as a Bluetooth car device or AirPods, appear as options when they’re connected to iPhone.”
Excerpt From: Apple Inc. “iPhone User Guide.” Apple Books.
https://books.apple.com/us/book/iphone-user-guide/id1515995528
in iOS15 there is no such option and instead these are the only options
“Change other settings for spoken directions
To change the volume, press the volume buttons on the side of iPhone.
In Settings , you can choose whether to pause spoken audio (like podcasts and audio books) during spoken directions and whether to wake iPhone for spoken directions. When iPhone is connected to a compatible car’s audio system, you can also choose to hear spoken directions while listening to the radio.
Go to Settings > Maps > Spoken Directions, then select your options.”
Excerpt From: Apple Inc. “iPhone User Guide.” Apple Books.
https://books.apple.com/us/book/iphone-user-guide/id1567104665