Domain Name Replaced in SERPs With Alias Domain Name

For one of my websites I have 3-4 additional domains which are 301 redirected to domain.com (they’ve always been 301 redirected). These alias domains are not just domain.net but also synonymA.com, synonymB.com.Turns out that if I do a Google search for “synonymA.com” the main website will show up, but instead of domain.com in the URL field Google shows me synonymA.com instead!