My brother is having the same issue, accidentally used the wrong email address, and now his steam account is permanently attached to this origin account?
Went through support 3 times, and spoke with a representative about deleting the origin account but it still hasn't solved his issue. The fact that it took a single login with no confirmation is just plain out stupid, not to mention I've been reading other users have had their steam Email automatically used when their origin account is a separate email address. That these can be paired but not unpaired is absolutely unbelievable. If someone had access to my account (which has happened before, thankfully I have 2FA now) they could just arbitrarily gain access to my Apex Legends account and literally just permanently have it? I'm hoping EA can do something about this ASAP, but for the time being it has killed his will to play and I no longer have my gaming buddy. We were very excited to play and now we're waiting around for something that should be any easy fix.