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:

http://confluence:8090/rest/synchrony/1.0/content/115900417/changes/unpublished

Having said that, if we add bottom rule to the web.config, the issue will be resolved:

<rule name="Synchrony Rest HTTP" stopProcessing="true">
<match url="rest/synchrony/(.*)" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false" />
<action type="Rewrite" url="http://localhost:8091/rest/synchrony/{R:1}" />
</rule>