Mystery of my mom's citizenship

I had previously confirmed I have German citizenship through descent. In short, my grandmother was born in Germany in the 1940s, she married a foreigner, moved to his country, and had my mom in 1961. My mom later moved to the US and had me in 1989 before naturalizing to the US in 1992, hence she was a German citizen when I was born and I am a German citizen by descent. Great right?

When I went to declare my citizenship by applying for a name declaration (with plans to apply for my passport after), I was told it is unclear how my mom is a German citizen. Here's why: As proof of my mom's German citizenship, I submitted a notarized copy of her German passport, issued in 1987 in the German Embassy in her home country. However, she says her first German passport was issued in the early 1970s, when she was around 12-13 years old, also at that embassy. At that time, it seems, she would not have been eligible for citizenship as her father was a foreigner. I know the law was changed in 2021 due to sex discrimination, but does anyone have any theories as to how she was able to get her passport in the 1970s/80s outside of Germany?

Thanks to all who consider responding.