hmm ... hard to be certain from the pix- are the brown areas soft ?
Some of my Onc hybrids, the older pbulbs turn brownish, but still firm. I also have some that got "cooked" from being in afternoon sun during heat spells that got brown sunken areas, but it hasn't spread.
If the areas are soft and spreading, it doesn't sound good, tho it doesn't mean the whole plant is in danger. (I have had pbulbs rot, but not spread to the rest of the plant)
I'm interested in what others have to say, as I frequently go with "wait and see" myself, tho I'm sure that's not always the best way to go