I finally got this working : It takes time for Apple's Bot ("AASA-Bot/1.0.0") to come and read the change. It can be a few minutes or even an hour from looking at my logs. I hope apple will add some sort of manual option to get the bot to read apple-app-site-association for development. After the Bot read the new conf in apple-app-site-association i had to uninstall the app from my phone and reinstall to get this working. Not sure why but this was the only way to get things working.

This is a MUST FIX. The sound is hardly noticible and it's not possible to send a push notification with a different sound as IOS ignores the request and always plays the new sound.

the wierd part is that for push notifications that work, firebase returns something like : /messages/1728474452425638 for me it's something like : /messages/0:1728475944752677%264053f7264053f7

after hours of trying everything i finally decided to delete the Test Flight app. After deleting it, i also deleted my app and reinstalled it from the AppStore and what do you know… push notifications started working again. The Test Flight app was probably holding something that also affected the installation of the app from the AppStore. before removing the Test Flight app completely from my iphone i removed my app and installed is from the AppStore several times and nothing helped. Only deleting the Test Flight App got the push notifications working again. no idea what can cause this.