I thought SSA stopped using gender markers, but Medicare still used them.
I found this at the
Advocates For Trans Equality website:
"The largest system used by private employers, the Social Security Number Verification System (SSNVS), eliminated gender in 2011. However, some systems used by state government agencies will still match gender against SSA records."
So that explains it. The earlier change (2011) only applied to the SSN Verification System, not the SSA records themselves.