You really should ask this in

http://discuss.grapheneos.org/

https://lemmy.ml/c/grapheneos.

It’s a very helpful community. Someone will either explain why or point you to a FAQ. I bet it has something to do with the built in reader being insecure.

Edited… I totally gave the wrong url. Sorry. Corrected.

I use android auto and this has been killing me forever… Until about 6 months ago. I found this app on FDroid. AutoNotify. I was never able to get Signal notifications on the speakers when connected to Android Auto. This fixed that. Check it out. It works.

https://github.com/zehuanli/AutoNotify

It works. Been using it without fail.