From my perspective, that appears to be solid bedrock. If it was not, those guys could not be walking around on it without sinking to their neck. The brown part you see is either a different type of bedrock (meaning that is a fault line of sorts between them) or it is dirt/fill that the concrete spillway was poured on.
Just a guess, but my simple explanation would be that I assume the ground moved and the fill settled some point after the spillway was poured, which allowed water to flow under and wash more out, then the unreinforced concrete slab that was designed to be bearing on firm ground, not spanning over a hole, cracked and when water started flowing over it and through the crack it just further eroded it almost instantaneously. I am amazed I don’t see any reinforcing hanging out, whatsoever, so the whole spillway appears to be just a mass of concrete, which just seems stupid to me in a highly seismic area like California.
But you are right, I think there is a cavern there, from water hitting bedrock and forced back under the spillway slab... I am betting that hanging piece of slab is now gone. Probably up to where it was on bedrock again and not on fill. I think this was a picture of the initial hole before it grew the first time? I bet it is quite a bit bigger now that they more than doubled the flow that caused the big damage, it got up to almost 120,000 cfs I think.
Saw reporting today discussing cavitation as one culprit in spillway concrete damage events with other dams. The fix has not been applied to the Oroville dam.