I think this might have something to do with calculatePriosIso() in MapRenderer.cpp. I changed this line:
```(*it)->prio += (((uint64_t)(tilex + tiley)) << 54) + (((uint64_t)tilex) << 42) + ((commax + commay) << 16);```
to
```(*it)->prio += (((uint64_t)(tilex + tiley)) << 48) + (((uint64_t)tilex) << 32) + ((commax + commay) << 16);```
I made the numbers mimic the similar line in calculatePriosOrtho() (no other thinking beyond that). I was able to use prio=0 for NPCs after doing this and noticed no flickering. I'm no good with bitshifts, so maybe @stefanbeller could offer some explaination?