Can't use (or choose) "Microphone port"

MacBook Pro (Retina, 15-inch, Mid 2015) with Catalina 10.15.5. I am having trouble with the external microphone port - it's not recognized in System Preferences.

1) In the past, (up until a couple weeks ago), the Microphone port and Headphone port worked flawlessly with my external headset.

2) Today, the headphone port works fine, but the Microphone port is not recognized in System Preferences, even when the external headset is plugged in.

3) Restarting macOS causes Microphone port to work, but unplugging and replugging the headset breaks the input (headphones still work). System Prefs shows input remains set to "Internal Microphone"

Here is a series of screen shots:

a) Immediately after reboot with external headset plugged in the whole time. System Prefs shows "External Microphone" and "Headphones" (Correct)

b) Unplug the headset - System Prefs shows "Internal Microphone" and "Internal Speakers" (Correct)

c) Re-plug the headset - System Prefs shows "Headphones" (correct) but "Internal Microphone" INCORRECT - the computer seems not to detect the presence of the headset jack.

Thoughts? Thanks.