
Wait, was that a bug? I always figured it was just based on how insanely difficult it is to keep cities clean as they grow massive. You can still easily hold on to those cities, even very distant ones, by recruiting lots of peasant units to garrison the cities. The security bonus is based on the number of men you have garrisoned versus the number of civilians, and since peasants are the largest units by manpower, they grant the biggest bonus. You wind up with two rows of peasants that are only useful as bait in an actual battle, but give plenty of security bonus to offset the max squalor penalty.
Edit, actually it gets even easier if you keep recruiting peasants as a sort of population control even after the garrison is full. Send excess peasant units to your most recently conquered cities to maintain control and free up militarily useful units from just standing guard, and for certain cities with super slow population growth you can disband the units as they arrive in order to boost the civilian numbers. It’s a makeshift, but effective way to transfer population from overcrowded cities to the empty ones.
Go big or go home. “Russia starts using nukes and us government blames ukrainians for it”