We just upgraded our Phabricator instance to the latest version this morning (11/24), a bug was reported:

If someone accepts a PR while you are editing it, your edits end up undoing changes they've made.

Example:
XXX accepted the PR while I was editing it.
When I saved my edits, he got removed as a reviewer and added back as a subscriber.
I manually added him back as a reviewer, but now there is no longer a green checkbox by his name.