This issue, it seems to me, is related to the fact that Facebook takes over control of the phones speaker when you open the app.
When you have FB open, the speaker becomes dedicated to FB.
This could be related to the “autostart” feature on FB that has videos play automatically when you scroll over them.
On my iPhone 13 Pro, the FB app if opened, will kill radio broadcasts and other sounds as well, as it takes over sole use of the speaker.
I have not played around with the “autostart” and other functions of the phone or the FB app.
But some of you tech guys here will know what I’m talking about and you should look in this direction.