Well that's very interesting! That would suggest that it was a driver problem pre 2004, when you updated the OS, it likely updated a new driver with the faults corrected.Some good news!
Yesterday, I updated Windows 10 to v2004.
A short while ago I was idly thumbing through the list of programs in my Start menu and saw 'Realtek Audio Console' which was definitely not there before.
I opened it up and went to the advanced settings where it allows you to be prompted as to whether you've plugged in a headset or a microphone (it's never prompted before) - but no headphone option. I quickly plugged in my earphones to be met with the prompt which had an option for headset or headphones. I selected headphones and went back to the settings to disable the prompt.
At last - I am able to use my laptop's microphone when the earphones are plugged in.
Thank goodness I didn't request a return!