The areoles and spines in the yellowish area look dead, now the areoles close to the dulling areas are turning brown, is it rotting or corking? The dull area does not feel squishy nor tough, the upper part of the cactus feels normal

How long have you had this cacti in that container?

It seems corking but there is an angle of your shot that looks like it is getting a softer spot. Sometimes I poke the soft spot to see if it is mushy or tough. Best thing to do is to remove from current container and further inspect the condition of your roots.

Depending on your location and where you are growing it whether indoors or outdoors, the plant might be getting ready to go dormant when temps cool down and light levels getting shorter, so be very careful with watering and make that media really porous. Need to adjust/lessen watering if you are heading into cooler temps.