Unfixable display scaling when Mirroring “unsupported” Displays

## What Happened
When I tried turning on the Mirror Display option for my TV, UI scale would increase by a lot. I would've turned off the option but I can't access the toggle since it's at the bottom of the window. My TV starts showing the "Unsupported Video Signal" screen with Night Light on and off, even though it works fine without Mirroring.

## Expected Behavior
When turning on the option, the displays would blink, and the external display would be affected while the main display stays the same.

## Steps to Reproduce (based off of complete assumption)
1. Open Display Settings
2. Connect a HiDPI display alongside a LoDPI display
3. Turn on Mirror Display

## Logs
Unavailable

## Platform Information
elementary OS 5.1.7 (Mostly up to date Switchboard)
Kernel 5.4.0-54-generic
Intel Core i3-3217U/Intel HD Graphics 4000
4 GB RAM
490.7 GB/500 GB HDD

Hoping this gets fixed in eOS 6!