Is this bud rot ?

Possibly, very hard to tell in those photos. It is quite obvious, I had it once just on one bud right near the end of flowering. I think it was one of the gggs from my early grows. It was a different colour to the rest of the buds, like browner and dull and looked dead and felt almost dry enough to "smoke". Just snip it off and check the other buds as thoroughly as you can, try to reduce your waterings and humidity levels is the only thing you could do