Is this a bud rot?

Cut below what you see. the first point you can find stem below that 'bud.' Pray it didn't take foot elsehwere, but it likely has. By the time you see it, it's already spreading. Dying sugar leaves are a hint it is underneath, unseen Do some math. Lots of plants, i'd remove the entire thing. Get it away from the bulk. If it is one plant, maybe you try to salve some with aggressive amputation. consider harvesting early if you do find more spots and need to amputate more. I would not mix this bud with anything else -- dry in separate area etc. I'd expect more spread than you can see. It's beyond the resolution of the human eye.