Custom field in IIS log has "-" for value for certain Paths - url-rewrite-module

Question

I have configured the following rewrite rule in an ASP.NET application's "web.config" hosted on IIS:
<rewrite>
<rules>
<rule name="setappname">
<match url=".*" />
<serverVariables>
<set name="CONTAINER_APP_NAME" value="desiredValue" />
</serverVariables>
</rule>
</rules>
</rewrite>
And in "applicationHost.config", I have the following snippets:
<sites>
<site name="mysite" id="1" serverAutoStart="true">
<application path="/" applicationPool=".NET v4.5">
<virtualDirectory path="/" physicalPath="c:\mysite" />
</application>
<bindings>
<binding protocol="http" bindingInformation="*:80:" />
</bindings>
<logFile directory="c:\iislog" period="MaxSize" truncateSize="4294967295">
<customFields>
<add logFieldName="x-forwarded-for" sourceName="X-Forwarded-For" sourceType="RequestHeader" />
<add logFieldName="container-app" sourceName="CONTAINER_APP_NAME" sourceType="ServerVariable" />
</customFields>
</logFile>
<applicationDefaults preloadEnabled="true" />
</site>
</sites>
AND
<system.webServer>
<rewrite>
<allowedServerVariables>
<add name="CONTAINER_APP_NAME" />
</allowedServerVariables>
</rewrite>
</system.webServer>
This works fine (I see the 2 custom fields in the logs) except when the Path ends with "/" (e.g.: / or /APath/). In those cases, the value of the container-app field (using Server Variable) is always "-". For instance:
$ curl --silent --output /dev/null -H "X-Forwarded-For:10.3.2.12" http://localhost/APath/
Yields:
2019-12-02 20:47:32 172.29.152.165 GET /APath/ - 80 - 192.168.7.4 curl/7.67.0 - 200 0 0 121 10.3.2.12,+::1 -
Whereas:
$ curl --silent --output /dev/null -H "X-Forwarded-For:10.3.2.12" http://localhost/home.aspx
Yields:
2019-12-02 20:50:17 172.29.152.165 GET /home.aspx - 80 - 192.168.7.4 curl/7.67.0 - 200 0 0 63 10.3.2.12,+::1 desiredValue
I even enabled the Failed Request Tracing to see if perhaps the rewrite rule isn't picking up those paths, but I can confirm that the rule matches the path and the server variable is set to the desired value.
I wonder if there is anything else I can try to troubleshoot this. Why such paths aren't logged properly?

Answer 1

I think I found the issue and posting it here for others.
By looking at the Failed Request Traces, I can see that IIS creates child requests for directories' default documents (URIs ending with "/"). Apparently, by design, rewrite rules don't apply to child requests (e.g.: https://forums.iis.net/t/1152699.aspx).
To solve this, I created a rewrite rule to change such requests to be explicit requests to documents so the other rewrite rule gets applied at the main process level:
<rewrite>
<rules>
<rule name="setExplictDoc">
<match url="(.*(APath)/$)" />
<action type="Rewrite" url="{R:0}Default.aspx" />
</rule>
<rule name="setappname">
<match url=".*" />
<serverVariables>
<set name="CONTAINER_APP_NAME" value="desiredValue" />
</serverVariables>
</rule>
</rules>
</rewrite>
The idea came from https://support.microsoft.com/en-ca/help/3050055/iis-digest-authentication-does-not-permit-pass-though-authentication-f