Bluetooth Broke Siri, Help Wanted.

I've been through the ringer on this problem, trying all the recommendations I could find on the 'net. Still, this issue vexes me, and it seems my problem is almost exactly the opposite of what most people report.

After pairing a set of Mpow bluetooth headphones to an iPhone 8 Plus, then unpairing and forgetting that device, Siri is broken on the iPhone. Sometimes...

With no paired Bluetooth devices active (i.e., just the iPhone), Siri does not hear or respond to voice commands. It doesn't even seem to detect input from the microphones (see below).

If I pair the Bluetooth headphones to the iPhone 8+ again, Siri works fine, both on picking up voice commands from the phone, and reporting output over the headphones, as expected.

If I pair the iPhone 8+ to my car's Bluetooth, Siri works via the iPhone and the car's speakers, as expected.

If I unpair or turn off the car; unpair the Bluetooth headphones; or turn off Bluetooth on the iPhone altogether, Siri goes deaf and mute. No amount of yelling commands into the mics can wake it up. For all intents and purposes, if I'm not using a Bluetooth device paired on this phone, Siri is completely dysfunctional.

All voice messaging apps, voice memos, and phone calls work fine and suggest that there are no problems with the microphones. I've done a backup, reset all settings, and restore with the same result. (Even after resetting and before restoring the backup, the problem persisted, as though it still persisted on a "clean" install of iOS.) I've turned Siri off and on between reboots. If the headphones are paired, when I hit the Home button, Siri shows the Bluetooth symbol... if I tap it, it suggests the option of choosing between Bluetooth and the iPhone, but nothing happens if I choose "iPhone". If there were other potential solutions on the 'net, they don't work either; I've tried most all of them.

The iPhone is quite new (about 4 months), and functions as expected in every other way. I'd hate to have to contact support directly -- I doubt I'd get a fix, from what I've seen -- and I think it would be wasteful to try to get a replacement iPhone, especially since I suspect a restore from backup would only reintroduce the problem. Closest "Genius Bar" is a 6-hour round trip away.

Does anyone have any insight that would be useful?

Thanks!