Subsite redirects to main site, unless https

Hello,
I have the following problem:
I have a network with a main domain (example.com) and a subdomain (foo.example.com)
The site is registered under the URL http://www.foo.example.com, and the A records for foo.example.com http://www.foo.example.com are all pointing to the WP installation.
But when I visit foo.example.com it redirects me to the main site.
If I visit http://www.foo.example.com it also redirects to the main site, the only option where I am not redirected to example.com is when I fully type out https://www.foo.example.com.
I think this is a URL rewriting thing, sorry I cannot share the URL due to safety concerns of the client, but what could that be?
I read that MU supports domain mapping (www.foo.example.com to foo.example.com, as an example) natively, but the articles where this is allegedly explained is dead.
Wordpress Version is the latest one.