Monitor automatically adjusts to 144hz refresh when Mirroring is disabled, going black.

Display: Z-EDGE UG27 monitor

-27" Curved, 200hz

Source Device: iMac

Not exactly sure which model/year, but it's currently at its latest possible update, macOS Catalina 10.15.7

Connections: DisplayPort cable > DisplayPort to Thunderbolt max 60hz adapter.

So I just hooked everything up the other day, and I was testing things out. I moved my system settings tab over to the monitor's display, and noticed I could change the refresh rate. I experimentally changed it from 60hz to 144hz, turning my monitor black, with the System Preferences tab still on it. To fix this, I ended up holding the tab with my mouse as I unplug and re-plug the thunderbolt adapter back in. The tab no longer shows Refresh Rate nor Resolution options, but still shows arrangements. Upon checking Mirror, an additional tab appears, allowing me to change the monitor's refresh and resolution, but only when in "Best for UG27" mode. Otherwise, refresh rate becomes greyed out. After changing the rate back to 60hz, the montior comes back on, and I can change it to "Best for iMac" again successfully, however it still greys out the refresh box.

I don't want to use Mirroring though, I would like to use 2 displays. Unchecking the Mirroring blacks out the monitor again, and when re-checking it, the refresh rate has returned to 144hz.

It seems that the problem is I can't stop the settings from changing back to 144hz after diabling Mirror. Is there a fix for this?