Exit stages of transition on modal don't seem to run

[jarcoal]

Describe the bug

When providing a transition in the Modal component, the exit stages of the transition don't seem to run when show is set to false after being true.

Reproducible Example

Here is a full reproduction: https://codesandbox.io/s/react-modal-transition-issue-5j4sz

You can inspect element on the modal to see that it never gets the ...-exit-active classes from CSSTransition.

Expected behavior

I would expect the exit classes to be applied, but the modal seems to unmount from the DOM instantly, not giving them a chance to run.