That's part of the functionality, you can kill enemies you've maxxed out on.
Is it? It seemed like a glitch to me. After killing the vampire, it said 1/1 and went red, and then I couldn't kill it again. However, after killing the hydra, it still said 0/1, and I could kill it again. That seemed unintentional. Also, I was able to kill other normal enemies a few times before they went red, and they would say something like 103/100 instead of 100/100. I can't see how this is anything but a glitch.
The UI doesn't update until you interact with it in some cases. One of those is when you're fighting and complete the fight. You can select the monster and continue killing it but you receive no reward since you've already maxxed the count out.
It's kind of a bug but that's likely based off the order they update data.