Nonexistent mobs?

I currently have a survivor hopeful in the southern Shiverpeaks. In the process of capping two dozen or so elites yesterday and today, I noticed that some of the boss spawn points were occasionally empty. It's not just that they didn't have the enemy I was hunting...there was nothing there, period. No boss, no minions, only vacant terrain.

I know that the spawn locations are correct; I combed each of the explorable areas multiple times as I was constantly swapping secondaries. I noticed that whenever this happened, it seemed to affect a single species throughout an entire zone. For example, I was in the Frozen Forest once and there were zero Pinesoul bosses to be found. Same with the nameless Mursaat in Ice Floe.

On to my question(s): is this a known bug? Am I just unlucky? Or maybe it's much more ominous than that...perhaps I triggered the anti-elite skill farming code?

Doesn't sound like anything intentional.

Nope sometimes they just dont spawn. It's perfectly normal and gl on the survivor.