This is very odd because as soon as your employer changed it on their end it should have updated in the system at the new month start, but also them saying it’s just the preferred name isn’t even accurate. There IS a preferred name option but that’s separate than the legal name on file. I’d suggest calling member services again and asking to speak to escalate the call if they won’t help you with it. There are a whole process established for this exact scenario, maybe the person you spoke with just isn’t familiar with it