app_offline.htm not working for rewritten URLs

app_offline.htm not working for rewritten URLs

I have a Rewrite rule for a specific page, to make it more SEO friendly.
<rewrite>
<rules>
<rule name="Friendly Threads">
<match url="Topic/([^/]*)/([^/]*)/[^[#\?]*\??(.*)" />
<action type="Rewrite"
url="thread_messages.asp?ThreadID={R:1}&amp;PageNumber={R:2}&amp;{R:3}"
/>
</rule>
</rules>
</rewrite>
But whenever I drop an app_offline.htm file in the root, the site will go
to the app_offline.htm page for all URLs except for those matching the
above match.
Is this expected, or have I possibly got something misconfigured somewhere?
If it makes any difference, this is a classic ASP site running under IIS 8
(Windows 2012).
(And yes, I know this is like putting a 90 year old behind the wheel of a
Lamborghini.)