Yep, it's not about the continent tier, it's about how many missions you have won in general. And where you look (interiors, undergrounds, or surface).
If you look on the surface, you'll just be getting boss towers at that point because that's all that can really show up there. If you look underground, it's just a few very early on in (rescue missions, fix the anachronism, etc). But pretty soon you start unlocking things like the meteor defense ones on the surface, boss gangs for interiors and undergrounds, umbra vortexes for interiors and undergrounds (or just undergrounds, I can't recall), lava escape for interiors and undergrounds, journey to perfection for all three, etc.
If you look at the "win X missions" unlockables, you can see what you get at each tier. There are special rescue NPC mission rooms underground and in interiors aside from just generalized secret missions, by the way; because the rescue missions tend to become very diluted in the normal secret mission rooms before long, as you're observing.
