Rewrite with RegExp to Move Page Doesn't Work

Hello,

We would like to move requests from one page, now non-existent, to another (e.g. site.com/pages/page.aspx?s=123ASDF to site.com/page.aspx?s=123ASDF)

It works with rewrite maps, added for specific querystring values:

<rewriteMap name="Legacy"><add key="/pages/page.aspx?s=7C3D7EAB6B2242A" value="/page.aspx?s=7C3D7EAB6B2242A" /></rewriteMap>

However, generic rule with regular expressions doesn't work:

<rule name="Legacy2" stopProcessing="false"><match url="^pages\/page\.aspx\?s=([0-9a-zA-Z]+)" ignoreCase="false" /><conditions logicalGrouping="MatchAll" trackAllCaptures="false"></conditions><action type="Rewrite" url="page.aspx?s={R:1}" appendQueryString="true" /></rule>

Expression tests correctly.  Requests stop here instead:

<httpErrors><remove statusCode="404" subStatusCode="-1" /><error statusCode="404" prefixLanguageFilePath="" path="/errorpages/filenotfound.html" responseMode="ExecuteURL" /></httpErrors>

Failed tracing reveals nothing.

Please help me understand what is going on.
Thanks.