Looking at the IIS documentation we can see 3 rewrite requests. For Synchrony (synchrony/(.*)) for Confluence requests ((.*)) and Synchrony Websocket requests (ws://(.*)).
In the case when we create a new page, save it, then again edit it and discard the draft, Confluence will make a bottom request which will not be handled by 3 rules from above:
Having said that, if we add bottom rule to the web.config, the issue will be resolved: