The answer is that they don’t literally go backwards in time showing reverse causality. They are representable in QFT by reversing the time component of their counterpart, which doesn’t impact how they interact with broader causality, just their internal configuration. Further - this does not play out in the lab as reversed causality even between anti particles. You can collide two positrons then detect them later at their destinations just fine in normal causality. It’s a shame because it would be really sweet to find some reversal of entropy and causality somewhere or somehow!