Steps to reproduce:

1. Create a terrible, bug-ridden revision with some set of reviewers.
2. Realise the error of your ways and run arc diff --update on $#*!:
   1. Apologise profusely and hope you don't get fired.
   2. But, then a shiny object appears, and you get distracted before exiting your editor.
3. Alice, not a current reviewer, approves the diff. This adds them as a reviewer.
4. Suspecting that shiny object was actually Dave playing with a laser pointer, return to your desk and :wq out of Notepad™

What should happen:
You regain your honour by fixing your code.

What happens:
Alice is removed from the diff