Hi Pascal, welcome to the community forum 
Could you indicate which OS and version you are running on the FP3+? This is found by going to Settings > About phone > Build number
The most recent update (for standard FPOS) is 8901.4.A.0010 & 8901.4.A.0011 announced here. This is a version of Android 11.
Any additional information you can supply might help (apps, versions, such as Android Auto?)
Rather than using Bluetooth you may obtain better performance using Android Auto Wireless which uses Wi-Fi (much better bandwidth likely required for full functionality). This is reputedly available on Tourans since 2016 and on phones running Android 11 and supporting 5 GHz Wi-Fi (the FP3 running latest FPOS update is compatible). It requires setting up initially using a USB connection as described here for example.