Ford Sync audio stopped working with iOS 18.4

I have a 2015 Ford Focus with a Sync II stereo. It has all been working fine for several years. Just recently, when my phone connects via bluetooth, the audio is no longer routed through the stereo, so I can't play music through it, and maps directions don't use it either. However, hands-free phone calls work fine through the stereo. The car reports the phone is connected, and it shows my phone's name, and battery and signal levels.

Note that this is not CarPlay; this car doesn't have that at all, just regular Bluetooth profiles.

Speaking of profiles, I can select from several different profiles, such as "Car Stereo", "Headset", and a couple of others, however, selecting any of those manually doesn't seem to do anything, and I've never had to do anything with that in the past. I have tried deleting the connection (at both ends) and re-pairing, but that didn't help.

It's not my specific phone because 3 of us all have similar devices and they all stopped working about the same time, so it's either a car problem, or a general iOS problem.

We are all running iOS 18.4.1 now, but the problem persists.

Any ideas?