I have the same issue with my hearing aids (new ones).
I can connect them to the phone wit the bluetooth connection. I can stream audio and hear incoming calls, the person on the other side cannot hear me. But after a while when the connection gets lost, and I reconnect, nothing works. The phone is registring my hearing aid in bluetooth, but no sound is getting transfered.
I can also connect through the amplifon app that my manufacturer is providing. The thing is that you connect the hearing aids and it works, but the app is telling you to restart your hearing aids and after that it doesn’t work anymore.
I also have this with other devices on android 14, (have tested on a device from my work). So it also might be a problem with android 14. I hope it is fixed in android 15, but I don’t know how long it takes to roll this out on Nothing Phone 2. I can test this if it is an update without a factory reset.
I have a Samsung Galaxy S24 tested today with android 14 and on this it is working like it should. So they have implemented it well, but on the Nothing Phone and other phone’s like Xiaomi it is not working for me. Maybe you can get in touch with Samsung on this?