Jason, how did you setup the rewriter in IIS. If you get a 404 error than it has to be an issue in the rewriter configuration.
Our documentation on: https://world.mendix.com/display/howto50/Set+up+Internet+Information+Services explains how to setup the url rewriting using the fusion rewriting library.
An important thing to consider here, is how you remove the /123/ from the original url. Because you don't want to include that part when redirecting to Mendix.
You can easily confirm and see what the exact issue is, either IIS forwards the url including the /123/ to mendix. In that case you will see the 404 message also in the Mendix log file.
Or there is no mention of the 404 in the Mendix log file, that means that IIS does not forward the deeplink url at all.
Either way you will have to take a look at the url rewriting configuration.