Azure B2C Owin redirect_uri_mismatch when using custom domain

UPDATE: I determined that because the B2C directory was setup with a custom domain name of ourdomain.com), there must have been an internal translation of that domain to the resource name (appname.onmicrosoft.com).  

myapp.ourdomain.com --> myapp.appname.onmicrosoft.com.

This was confirmed by sending that redirect URL in the request, but configuring the B2C application reply URL to myapp.appname.onmicrosoft.com.  There was no problem that the parameter didn't match up to the reply URL configured in B2C, but obviously I simply received a 404 since myapp.appname.onmicrosoft.com doesn't exist.

By creating a brand new B2C tenant/directory, and using only the appname.onmicrosoft.com resource name (no custom domain on the directory), the original problem has been resolved.