There is no change in the group of people older than 65, but there is a collapse in numbers in the group of people between 15 and 65 years old decreased. In the latter group, death rates are low. The health index is at 151+ and the welfare level is 132+. So, it's still a mystery to me, except for the fact that the population drop always appears to happen right after having set up disaster relief resources.
If you want (and have time for it), you can check: Republic of Selene, on KB. I would really like to know what happened, and I would be really grateful if someone could help me with it.