[DoctorOetker]

Initially I thought the same for a couple of seconds because of mass energy equivalence, but then I realized:

assume the fission event to occur in empty space, assume the liberated energy is isotropically radiated, and assume all the energy is thus expanding spherically, then the mass/energy that was initially in the mother isotope is now after some time distributed on a spherical surface.

As long as the observer is outside of the shell of escaping energy, to the observer the gravitational field looks like all the mass is concentrated at the center of the expanding shell of energy, exactly where the mass/energy originally was, and hence gravitationally undetectable.

After a while the expanding surface has past the observer, and the observer is inside a spherically symmetric shell of mass/energy. Even if the observer is not at the center there will no gravitational field at all according to the observer. (consult any physicist or physics textbook, or do the exercise for the gravitational field in a 'hollow earth'). So to the observer this looks like a bit of disappearing mass at the location of fission event.

It seems extremely difficult to me to actually measure such an event gravitationally though..