This is from my experience, but what I've noticed for that specific raid, Cublrac does always spawn but the timing is strange also the location is sometimes different.
I'm not 100% sure but I have seen him in 4 different spots when that raid is active, but a possibility is that when people run in towards the stairs, he could've simply followed and moved south (within the gates). But the other locations were outside the gates just east or west from the gates itself. So there's a possibility that he does spawn but nobody found him to kill him (not a lot of people like to kill the orcs, they just run for the bigger bosses, and he looks like an orc marauder so unless they notice his name, most people wouldn't care to check). I don't think he would "de-spawn"? (not sure if that's the correct term) if nobody killed him during the raid time, he would just poof but if bosses do that then there's a chance that could've happened too.
As for the strange timing, I noticed that sometimes it could be after the last message (when Zulazza spawns) that he spawns or somewhere before the gates open (which is the few times I found him outside the gates.) I keep trying to go for that raid whenever I can because I want to see if he appears more than once during that raid. I know there's a chance in the other raid for him to spawn multiple times. So my theory was if he spawns in different locations and not always the same time, could it be that he might spawn more than once? Something that I'm still trying to figure out, sometimes people leave the raid after Zulazza dies but there's still creatures around, I haven't been lucky to find another as yet so for now it's not proven but I'll still keep checking whenever I get the chance.
I hope this helps!