Donk! trophy unlocks but the same objective in combat challenge menu doesn't?

[PondNewt]

As the title suggests, I've had an issue with the game where I was able to unlock the Donk trophy without a problem, but the same objective listed as part of the combat challenges didn't get ticked off.  I've since repeated the action many times with various different weapons landing the killing blow, but the relevant box under the combat challenges remains stubbornly unticked.  I suppose there isn't much to be done other than just starting a complete new playthrough from scratch on a new save file?   

 

Adding to my woes on a sidenote, I also encountered a glitch in the missing person Steve Lost & Found quest.  Unfortunately for me, the last of the three named costume type zombies wasn't correctly dressed for me in keeping with what Steve wanted, this happened even though my character spoke her line on it spawning as if it was the correct one.  When I then led the zombie back to Steve, he wouldn't say his line to give me the greenlight to kill it.  As such, I couldn't complete the mission.