The flash of inconsistency you see is because the client eagerly modifies state before the server persists it (or some form of this problem).

The routing issue is because the application (SPA) is using business logic to control the browser's navigation state. Sometimes opening a modal will be a new push on to the nav stack so you can hit back, sometimes it isn't, sometimes they forget things, etc..

All of it though is just too much state being managed on the client by the application and that state being managed poorly. This isn't only related to React. It's always been possible to screw up with managing state in any framework.