-1 happens when we ramp down a cache node.

A lot of the things you found weird is because of the ramp up/down feature, and a function (I removed in this PR) that dynamically resets the capacity of a node. I removed that function since we don't need the dynamic capacity change feature and I want to keep the new code as simple as possible. However, it left all sorts of weirdness in the code.

Yes, without dynamic capacity change, we will never see -1.