Mono audio forced in headphones (and other strange effects)

Issues:
Whenever I play any audio, the sound is distorted. Specifically, instrumental music often echoes, as if the computer is simulating audio in a cave. Some instruments are also muted. Any voices are either muted, distorted (like a bad connection on a phone call), or made much quieter, as if they were talking or singing on the other side of the cave.
These issues started yesterday, I think. I had recently moved my computer to a new location, but everything was plugged in in the same places.

These issues disappear when I set the left-right balance slider all the way to the right (or the left) in Realtek HD Audio manager. (See pic, bottom left corner)

This does not make all the audio play from one side, instead it plays audio on both sides of my headphones. However, the audio played is actually the right channel, which can be confirmed by playing a test sound in either ear. Essentially, it forces the right channel to play in both ears.

I really enjoy having stereo audio when I listen to music and videos, could someone please help me fix this?

Attempted fixes:
I have found many methods of fixing this through Google, with little success, including but not limited to:
I have turned off all sound enhancements from sound settings in Control panel (it wasn't on to begin with).
I have turned off mono audio in the accessibility settings (it wasn't on).
I have gone through Realtek HD Audio Manager and reset to default/turned off sound effects, equalizer adjustments, environment simulations, karyoke effects, headphone virtualization, and everything else that I could find (none of them were on).
I've updated Windows to version 1909 (see pic)

I've gone into Device Manager and disabled and re-enabled the default output device.
I've tried making Windows update the audio driver for the default output device, then I updated it myself by uninstalling the driver and installing a new one from Realtek's website.
I've tried using earbuds instead of headphones.
I've looked at every input device and unchecked the box that says "Listen to this device" (It wasn't checked).
I've gone through the default sound troubleshooter 3-4 times.
I've restarted the computer many times during this process.
After every method, the problem has persisted.

Conclusions and Speculation:
I wonder if Windows is trying to simulate some kind of surround sound, and force it into my headphone jack. If so, I don't want it. It might be a problem with the registry. I'm not confident I won't screw anything up if I touch the registry without guidance, so I came to this forum in case someone smarter than me knows what's going on.

You only mention headphones. Do you have PC speakers plugged into the rear speaker jack? and are they having the same problem as the headphones?

Fireberd, thank you very much for the reply. I did not have speakers plugged in originally, but after reading your post I found a pair of USB powered ones (Carise brand) and tested them on both the front and back audio jacks.
I had the headphones plugged in front at first, and when I connected the speakers to the back, only the right speaker worked. I unplugged the headphones and then only the left speaker worked.
I moved the speakers to the front jack, and my computer didn't seem to recognize they were there (I have front panel jack detection enabled in Realtek HD Audio Manager) while the speakers played static.
I moved the speakers to the rear jack again, and now stereo is functioning perfectly on the speakers without any distortion whatsoever. Both speakers are playing their own proper audio channel.
And now I've replaced the speaker with the headphones in the rear jack, and stereo is functioning perfectly on the headphones.
I told Audio Manager that the headphones were "Front Speakers," so that might have done it.
I did try that jack before, but maybe it needed to see actual stereo speakers before it would work with my headphones?
Anyway, I restarted and everything's still fine. I'm going to mark this as solved. Thanks for your suggestion!
I hope this is helpful to anyone having the same problem in the future.

Great! glad I pushed you toward finding the solution.