I've noticed a bug where population isn't created in rural areas of a map - i.e. it is there only in cities and towns that are actually on the map. This becomes clearly visible with the population map filter enabled. Affected areas as of version 9.1.18 (that I found):
1940, 1941, 1942 sandboxes
Yugoslavia + north eastern Italy
Czechoslovakia + northern Hungary
east Ukraine, Crimea, east Belarus
All central-asian republics - KAZ, UZB, TKM, KYR, TJK
2017, 2020 (div states, MAGA):
1936, 2020 (GC, Shattered World, World) seem to be unaffected by this.
I've also noticed that population of Crimea is below expected - as of 2014 census (source
) it stood at 2 284 769 (58% urban, 42% rural). It is around 1.4M in the game in modern sandboxes.