I have experienced this, but it doesn't seem to happen every single time...
A goblin scavenger followed me upstairs. I attacked him immediately, striking and killing him in one hit. He didn't disappear immediately. I stood on the spot (pressed '5') repeatedly, and he remained for another 29 turns before disappearing.
Next I lead an orcish warrior upstairs with me. I attacked him immediately, but missed him. He didn't attack back immediately (maybe just slower than me, but probably 'frozen'). I moved away one square and he remained in the same spot. I moved back next to him and pressed '5' repeatedly. After a total of 27 turns since I attacked him, he finally responded and began attacking me. His behaviour from then on seemed normal. We exchanged blows and when I killed him he disappeared immediately (as normal).
Strangely though, I lead a giant rat upstairs, attacked it and killed it in one hit, and it disappeared immediately without any of the above weirdness... Very strange! :)
Edit: This is in JADE 0.1.2