I was looking at a list of countries and their population on Polish Wikipedia. (It provides data for multiple years which I couldn't find in English.)
I noticed one quirk: between 2007 and 2008, the population of Bhutan dropped from 2,327,849 to 682,321 people.
The only information I found that happened was that around that time, a king abdicated and the first parliamentary elections were held. Not much more. I suspect something must have happened.
What is the reason for ~70% decline in population around this time?