Missing referer in header but only for MVC/Core applications

Yesterday we moved our test web server behind Big-IP (with SSL offloading) but when we did that our MVC (and .Net Core 2) applications don't have the referer in the header (not empty but totally missing). The web forms applications work fine and our production environment works fine with MVC/Core applications. The only difference is that in our test environment you go to a HTTPS website behind Big-IP to login but after you login you are redirected to a HTTP website that is not behind Big-IP. Does anyone know why this is happening? Is it Big-IP or something in MVC/Core?