Another Redirect to home

I’ve read through the other threads with the same issue, but I can find a solution that works.

I’ve got WordPress installed at SiteA.example.com and I’m trying to get SiteB.example.com to load SiteA.example.com/pageB but it always redirects to the SiteA.example.com homepage.

Both the HTTP_HOST and SERVER_NAME variables show the correct domain, I’ve disabled all other plugins and I’m using /%postname%/ permalinks (which work correctly).

Do you have any ideas? This is the exact functionality my customer wants.