I think I figured it out.
Recently, I've been keeping the Pogonomyrmex under very close observation while trying to disturb them as little as possible. For the longest time, I couldn't figure out why they were dying off. I think I did today, however. For some background, about two weeks ago I saw a worker in the outworld seeming to be struggling in a test tube with water. I wasn't sure why, so I pulled the tube out to do a closer inspection. I realized that a tiny strand of cotton had gotten entangled in one of the worker's legs. A couple hours later, I came back to find her dead. I didn't think too much of it at the time, just thinking it was a freak coincidence. I noticed it again today, and it got me thinking. What if the cotton is what's causing the deaths? I surveyed 15 dead ants from the outworld and closely inspected them for cotton strands. It is incredibly hard to see, which is why I haven't ever seen it before, but an astounding 12 out of 15 dead workers had small bits of cotton entangled in their limbs.
I promptly pulled out all of the remaining cotton from the outworld, and I'm going to finish cleaning it out after I finish posting this. Another peculiar thing is that a lot of the dead workers seemed to have a few eggs stuck in the cotton, which is wild to me. Not entirely sure what's going on, but hopefully after cleaning everything up, and removing cotton, it might help...
I'll keep this updated because I am intrigued to see if this is the problem. The one thing that makes me doubt it is that for the past month or so, there hasn't been much brood at all, just some rogue eggs at times. Any advice would be appreciated. Thanks!