This has been a reoccuring problem with many spawns. As far as I know, the GMs are aware of it and attempting to fix it. Although at the moment, I assume they are taking care of the Sonytards.
This is also true for the Undead Mage in Kaladim, the Assasin/Swashbuckler in LGuk and a few others. The named, nor it's placeholder will spawn. Thanks for telling me about the Berserker though, as I was headed there when I got off work.
Although, In response to the Ulger, I killed him twice last night, 12ish am and 12:40ish am EST. Got a few PH's, and 2 of him.
Its probally something minor in the code and I assume will be fixed soon.