This broke recently, but I can't tell exactly when. Maybe with the latest update? It's hard to tell (I hadn't played on the realm for a while).

Anyway, as the title says: pillager captains no longer spawn in outposts. Regular pillagers do, but never a pillager captain. We first discovered this because our raid farm broke. I verified if it was an issue with just that outpost: I tried the same on another outpost and it had the same behavior there. 

I also tried in another realm and in a standalone world, and it worked fine there. I did try creative mode in both worlds, and our realm is in survival/hard mode.

This used to work, but no longer does, and it seems to be a realm specific issue. I realize that this probably doesn't help much (not sure if I can pinpoint you to the actual realm), but this seems to be the only way to report the bug.