An old bug (2015/2016) seems to have resurfaced for me today.
Color representation in the material browser is inaccurate. This seems to be the case for certain materials only. I cant think of anything unique about these materials.
There’s the potential that this was first set up as a Texture image material and then later that file path was unchecked, to change to a standard (color only) material type.
Sound familiar to anyone?
Adjusting the color sliders by a few increments doesnt do anything to change the thumbnail representation, but changing them by a lot will seem to force the thumbnail to reset and become accurate again.