I have now on two separate occasions killed zombies - where the zombie falls down dead, only for the zombie to continue to make sounds and then for a "ghost"/invisible version of the zombie to attack me.
Seems to be resolved by shooting the dead zombies body once again, which kills the "ghost". I have two videos that can demonstrate this.
The first is here: https://www.youtube.com/watch?v=8WShJlB4Twc&t=7s