The only one i encountered as nonkicable is neco-arc, so i guess your game just glitched, also u can basically kill the antifurry in the morning after letting em in, they dont instakill after getting into your lab
did a little more research on the case, results shows that sometimes fursuiters may fake 2 tests, neutral result is not a confirmation of either it's a real furry or impersonator, so you should read whats going on during cutscenes of other tests, cuz theres hints 4 you to detect if it's an impersonator or not, tested on mobile, about 40 reruns.also side notice in one run i let Isabelle and malo first, they both always are sicripted impostors, and i can say that they both didnt kill anyone in a 3 day even tho I was letting in real furries inside, so of your case, I'd say that you've let some impostors a few days earlier than you've let another character inside, they faked 1 or 2 of your test, or you just skipped the text with details which said that theres something wrong with 'em, and you've thought that they're ok, that all to conclude