Slow spring animation is cut off after a while

CodeSandbox reproduction of the bug
https://codesandbox.io/s/framer-motion-border-radius-layout-issue-forked-s4ymf?file=/src/Modal.tsx

Steps to reproduce
I am not 100% sure, but this does not seem like a smooth transition to me. Of course it could be because the config values are just too extreme, but it could also be a bug. This is the transition config:

const transition: Transition = {
  type: "spring",
  stiffness: 4,
  damping: 35,
  mass: 0.5
};

1. Click one of the blue squares
2. Midway during the animation, it suddenly accelerates to snap to the end

Expected behavior

The transition should keep an even pace until the end.

Video or screenshots

https://user-images.githubusercontent.com/159500/124361203-260e2680-dc2e-11eb-9cd1-05f5ef7fafea.mov

https://user-images.githubusercontent.com/159500/124361207-2ad2da80-dc2e-11eb-9511-2db4ff4f8523.mov

Start by running the linked CodeSandbox reproduction and inspect src/Modal.tsx with the stated spring transition values. Reproduce the mid-animation acceleration, then trace the relevant motion behavior in the repository; done means the transition no longer snaps prematurely while preserving the reported reproduction.