OK, I retested this the morning and the only thing I found out:

I should stop playing M2TW after coming home from a party!

Sorry, was a little bit drunk and it looks like I repeatedly chased the same Faction Leader through the desert while killing off his Family Members with assassins.

So what I thought to be diffrent leaders was always the same one.

Sorry again. Next time I will double-check while sober before posting any wild theories.