when you go over the kill by 2 times the required quantity, the counter will reset and add 1 to the total kill required.
kill 50 abc monsters.
0/50 kill counter.
you kill 50, quest says complete (50/50), but then you proceed to kill 50 more, the quest resets kill to 0/51.
now u hv to kill 51 to complete the quest. you kill 51, quest says complete (51/51), but then you proceed to kill 51 more, the quest resets kill to 0/52.
this continues onwards.
i can replicate this results on 1 char, but not on another. not sure why
suggestion: you should put a cap on the kill counter and not bug out like that.