This just happened with a Fairphone 5 when calling from the car. After the connection was made, for about a second the sound came via the car speakers (as it should), but then it switched to the ear speaker of the phone (as if on the ear, not speaker phone). The phone showed the car Bluetooth as output. Switching the output to phone speakers and back to Bluetooth car connection did return the sound on the car speakers, but only for a seconds before switching to ear speaker again.
This car system never gave issues before.
Stock Android 13 FP5.TT3Y.A.127.20231122
Is this a known issue? Can I provide more debug info?