Why is Mapped Domain for a Subsite 301 Redirecting to Main Site Domain?

I just installed Domain Mapping for the first time, and it's not behaving quite right. The mapped domain for the subsite is 301 redirecting to the domain for my main site.

So, I can get subsites to work like this:

http://user.mydomain.com

But if I map a domain to that subsite, the mapped domain 301 redirects for some reason to:

http://mydomain.com

The first question I'd ask: what should I be putting in for "Front end redirect should be"? I may be messing that up, although I've tried most of the options.

Also, I don't know if this is relevant or not, but I'd like to force people to log into their websites through their subdomain login, so that I can use a wildcard subdomain SSL certificate for encrypted dashboard access. (This isn't set up yet.) E.g.,

https://user.mydomain.com

But the user's website would still be accessible at:

http://userdomain.com

So, I checked "original domain" for Administration mapping and Login mapping, if that's relevant.

Thanks in advance for your help!

Jacob