Well, this is gonna sound incredibly stupid but I just found a fix for this, sort of, and 100% by accident. When I turn off my 2nd monitor, in the Nvidia control panel, instead of defaulting to the PC Resolution group of resolutions, at 1920x1080, it instead goes to Ultra HD 1080p which stops at 120hz.
That is what it defaults to. However, scroll down in that list and it gives me the usual resolution and refresh rate.
I feel very stupid right now. But yet again, I do not understand why this would even be a thing.