This is a bug with the mobile site (as viewed on a browser, not through the app) using the moderator tools. Was able to reproduce on iOS 9.3.2 (Safari), but not on Android with either Firefox or Chrome.
When you go to a post with a flag and a review history, if you click the
mod menu at the bottom of the screen followed by
show flag history you get taken to the (non-mobile styled) flag history page.
On Safari only, when you hit the back button, the mod menu remains open. If you select
show review history from the open menu, the URL gets all jumbled and you end up getting a 404. See (Japanese) screenshot:
It looks like the show-flags is appended prior to show-reviews, hence the 404.