I'm not a geologist, but... Didn't the main spillway failure erode some of what was considered "bedrock"? Isn't the bedrock somewhat more erodable than the term "bedrock" would lead you to expect?
When you look at photos or drone videos of the erosion near the main spillway, it's clear that an immense amount of rock has been eroded away (the color changes from brown to gray). [2]
Juan Browne discusses the type and strength of the rock in one of his excellent videos about the Oroville crisis. [3] (Note that he demonstrates the weakness of the rock where he is standing, which is across the river from the spillway and might not be the same rock.)
[1] http://www.mercurynews.com/2017/03/03/spoil-sites-for-orovil...
Another wild guess: the main spillway hasn't eroded any bedrock, just concrete, dirt, loose rocks, etc.
If anyone actually knows versus my wild guesses, I'd be most interested.
Yes, bedrock is not indestructible, it could get eroded from water, but it takes times. Plus it get eroded slowly over time. The main concerns was the use amount of water released by the failure of the spillway. For sure the water keep flowing over the collapse spillway would erode the bedrock lowering it, but at a slower pace...