Hi Sean, I am not sure, but maybe it’s your old iMac. It looks to me like a Graphics Card problem (are your drivers updated?).
The only way I could get your effect, was in Solid Mode, with the Lighting set to Flat.

But in Material Preview mode, the Cube is always blue.
Your Cube is not only white, but also has ‘flat’ shading (you can’t see the shape).
You also have some problems in the Preview area of the Material Tab (where it shows the material on a sphere, with the checkered background), where the shadow isn’t drawn correctly, so, I do not think it is you, doing something wrong, but your iMac…