Much anti-convert sentiment comes from the fact that our existence refutes the race science and blood quantum that has been built up around the Jewish identity in the last 130 years in order to justify Zionism.
Many Jewish people's sole connection to their Jewish identity is this genetics-based race science that they've forced to link them to Israel under the guise of being "Indigenous". What happens when that is debunked? What are they left with?
They see anyone who threatens this construct of phrenology as a threat to their Jewish identity, because what they've done is built their entire "self" around race science in effort to justify settler-colonialism as a project for the claiming of their homeland.
So, when converts object to Zionism, they call us "fake Jews", because our identities don't surround the settler-colony of Israel through the perversion of genetics. We threaten the security of the way they've manufactured their Jewishness to fit inside of a test tube.