Extrapolating from the geir qatan… There is a determinate state,
the problem is we only know it lemafreia. In the case of geir
qatan, we rely on the rov that end up accepting the religion they
were raised with. So, until you know the state, you can assume
they were megayeir. But if it turns out they reject the qabbalas
ol mitzvos, then they never were Jewish, and everything done
has to be reassessed accordingly.
So, for example, a qetanah who gets married to a kohein may
eat terumah. But if when she becomes a gedolah she rejects
her geirus, then that was in error, and they don’t need a
gett.
-Micha