Here they are in all their glory. What you can see here: a large area is cored already, stability +3, only one rebellion, only one war despite badboy being through the roof (by the way, I thought they stopped annexing/taking provinces shortly after the limit?!). What you cannot see: fully westernized, with western troops as well, second in tech behind me, more cores (basically all of China and central India) to come within the next couple of years - if it weren't 18something, this would be a formidable endgame opponent, they even have a larger army than me (largest in the world), although with my vassals I outmatch them.
Since I've been watching this for a while, here's roughly what happened. The original culprit wasn't Bhutan but Arakan, another SE Asian minor:
No idea how this came though. Apparently their tech lead gave them an enormous advantage against other nations of the area (even without better troops, I think Asian troops stop being progressed after a certain level, they have a better economy, higher morale, more units without WE and so on) and they became a mid sized power around the Gulf of Bengal. Then Bhutan revolted, apparently inheriting Arakan's tech level, and possibly coring all the provinces. Then the two became a bit gamey: one taking a few new provinces, the other revolting and taking over (and probably losing badboy and gaining cores in the process, maybe even new stability after westernizing decisions, though I do not know whether this is true). This happened a couple of times until Bhutan broke the "treaty" and stayed for good. I don't know when they westernized, but Spain and the UK were in the area.