General Relativity has identical solutions for forward and reverse time.

The Schwarzschild metric for example is expressed in terms of time squared so time itself can take positive or negative values and give the same result. It also allows negative mass but those solutions are not symmetric with positive mass - the rs term that contains mass is not quadratic.

One consequence of this combination is if you consider "negative mass" to be just positive mass operating in reverse time (to avoid having to accept the ugly idea of negative mass in positive time), you end up with asymmetric effects.