Walking Dead achievement works with 3 bugs. 1 bug is that sometimes the fight phase is replaced by the restart phase of the round. 2 bug is that after 1 bug the player remains a zombie and he has no weapons. And the 3rd bug is that if you write a kill to the console, then you will have achievements related to killing a man for a zombie
I will address the issue with the achievement in the next patch. I'll try to investigate the other "bugs" as well. Thanks for your report.
I have never seen how the event would be buggy or somehow achievements became impossible, maybe it's just you have problems