No one seems to have brought up your use of mineral oil.

Regardless of your problem, mineral oil is a 'wet oil'. It is used for cutting boards and the like because you never have to worry about working through a hardened finish. It does not fully dry or cure when exposed to air. It washes off over time and will need to be reapplied. You could possibly do a whole counter in this, but usually not.

A drying oil forms a hard protective surface when exposed to air. Drying oils can be used for furniture finishes, etc, where you would never use mineral oil. Think of it like a thin varnish.

Your countertop came with a protective finish. You sanded it off and then finished with something very different that won't dry in that section. I would not doubt this IS related to your problem, causing the color difference, but just in general, don't mix wet and dry oils for finishes.