There seems to be a bug when owning a Cambion Conception's Dark Bum and then picking up another one for the transformation: Both Dark Bums disappear. Might need some testing to confirm.
Binding of Isaac: Rebirth Wiki
There seems to be a bug when owning a Cambion Conception's Dark Bum and then picking up another one for the transformation: Both Dark Bums disappear. Might need some testing to confirm.