In my opinion, you just haven't been lucky enough to loot a given item, so you don't have the loot unlocked completely.
I will give you an example:
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I've killed 6 Yeti, Bunny slipers didn't fall out and the loot is blocked.
The same applies to War Golem, when I was making the bestiary I was not lucky enough to loot Tin Key and I also have loot blocked.
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So, in my experience, at least once you have to drop an item to fully unlock the loot.