SUMMARY
This is a very complicated bug for me to describe as I had to juggle and test three different displays, but I will do my best :)

When I power off my 165hz monitor and power it back on again, the displayed refresh rate in the Display Configuration settings changes from 165hz to 60hz. However, the monitor is running at a higher refresh rate - I assume still 165hz. This visual bug occurs in several scenarios that I have tested, including:
- Having display settings open before I power off the monitor. Here, it is listed at 165hz. When powering the monitor back on, it is displayed as being at 60hz, however it is *not* running at 60hz. This is a visual bug.
- Opening display settings after powering the monitor back on. The monitor is still running above 60hz, but is displayed as running at 60hz. A visual bug but occurs even if the system settings are opened after powering the monitor back on.
- Closing and re-opening the display settings in either of these two scenarios does not make a difference - The setting is still set at 60hz even if the monitor is running above 60hz.

On top of this, if another display is powered on and connected, the refresh rate does indeed change to 60hz. When powering on another display (in this case they are both 60hz), the 165hz display goes black (just the same as it would if one were to change the refresh rate), and then the refresh rate does indeed drop down to 60hz. Changing the displayed refresh rate does not make a difference, once the second monitor is powered on, it drops to 60hz. If a third monitor is powered on, the 165hz display will go black (but not my other 60hz display), however the refresh rate will not change.

This also applies to 60hz displays to some extent **when at least one other monitor is powered on**. If no other monitors are powered on, the refresh rate is unaffected **unlike the 165hz display**. If I set one of my standard 60hz displays to 30hz, when I power it back on, it will be displayed as running at 60hz but it will be running at 30hz. If I power on another display (either my 165hz display or my other 60hz display), the monitor will resume running at 60hz and the setting in display settings will correctly display 60hz. The monitor also goes black when this third monitor is connected, just like the the 165hz display does when its refresh rate is being updated.

If no other display is connected in the above scenario and the display only has two refresh rate options, **there is no way to correct the refresh rate to set it back to 60hz**. The display will be running at 30hz, but it will be set as running at 60hz. If I select 30hz, I can click apply, but nothing happens. The display does not turn black as it usually does when updating the refresh rate. If I choose 60hz again, I cannot click the "Apply" button. I have to power on another display, which forces the monitor back to 60hz.

Also, If the "Refresh Rate" setting is changed, but the "keep display configuration" dialogue is left to time out, the setting will incorrectly list "165hz" when the monitor is in fact running at its previous refresh rate (e.g., 60hz).

Finally, in my tests of powering displays on and off many times, there was an instance where after powering on another display, my 165hz display's resolution was set incorrectly, and the refresh rate setting was blank. I was able to click on it and it listed only 60hz and 50hz. Powering the 165hz display off and on again resolved the issue and I was able to set it back to 165hz.

There may be other scenarios I have failed to document, I tried my best to find as many details about this bug as I could.


STEPS TO REPRODUCE (Assumes a minimum of a 3 monitor setup)
165hz display scenario:
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1. Power off all displays.
2. Power on the 165hz display.
3. It will be listed as running at 60hz, however it will be running at 165hz (or whatever it was set at before it was switched off).
4. Power on a second monitor.
5. The refresh rate of the 165hz display will drop down to 60hz.
6. Power on a third display
7. The refresh rate of the 165hz display will not change - That is, if you left it at 60hz, it would not change, and if you set it to 165hz, it also wouldn't change. The setting in system settings will not change in either scenario,

60hz display scenario:
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1. Set 60hz display to a lower refresh rate, in my tests I used 30hz.
2. Power off the display.
3. Power display back on.
4a. If no other display is available, when powering the monitor back on, it will keep its refresh rate, but it will be displayed as running at 60hz. If there are only 2 refresh rate options, there is no way to change the displays refresh rate.
4b. If another display is available, when powering both monitors on, the 30hz monitor will change to 60hz and the setting in display settings will correctly display 60hz.


OBSERVED RESULT
Refresh rates of monitors are displayed incorrectly in system settings and set incorrectly when powering them on and off, in both single and multi-monitor setups, with particularly erroneous behaviour in monitors with refresh rates above 60hz.


EXPECTED RESULT
Refresh rates of monitors should be displayed correctly in system settings and kept as they were before powering them off.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux w/ KDE Plasma (5.15.12-arch1-1)
KDE Plasma Version: 5.23.4
KDE Frameworks Version: 5.89.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
- Unsure if this affects X11.
- There are various Plasma Wayland multi-monitor bugs, so I am unsure if this covers more than just one, but all of the issues I have described seem related to the same core issue of refresh rates not displaying and persisting correctly when they are powered off and on again.
- Did not test with more than 3 displays.