I have encountered the same issue.
In my experience, it seems to be related to how far the boss moves.
Keeping the boss close to where he spawns (within 20ft or so) properly gives you credit. If he is moved further than that, it tends to not give credit.
Additionally, his respawn timer seems to be really long. I've waited more than 30 minutes at times, without him spawning. Is that intended, or could it be bugged?