For the record:
In recent years, Israeli Chief Rabbi Goren (not chareidi by any standards, and quite a questionable rabbi) was the one who began annulling conversions which he didn’t like.
This story comes from the Langer siblings. Their mother was jewish, and she was married to a non-jew in Ukraine, who converted to marry their mother. Then, they divorced–without a get–and she remarried and gave birth to the Langer siblings. They came to Israel, where the rabbanut declared the mamzerim.
There was a big popular and political uprising against the rabbanut. Eventually, Goren ran for Chief Rabbi on the platform that he would be mattir the Langers. He won, and did so.
The theory he used was to invalidate the geirus of the first husband.
So, RD: which way you want to have it? Are the Langers mamzeirim or is the guy not jewish?