Anyone else think the concept of karma is inherently flawed?

Karma basically states that over time, a wrong that is done to you will also be experienced by the guilty party. this implies that there is a balancing force at work.

so here’s the false scenario;
if jack breaks tom’s leg, how are we to know that jack deserves any bad karma? what if he was simply an agent for this balancing force, because tom had wronged alex a week before? in this instance, jack should be rewarded.

no believer in karma has ever provided a compelling answer to a scenario like that.