I doubt it, it was never like that. The game takes it as all military ready manpower retreats to a free zone while the rest of civilians remain in the city. That would be mostly historical - if a town is about to fall, all people fit for service would go away.
Well, not all. Depends on your politics that is shown in the laws section.
For instance, as Germany you have strong constriction laws that give you more manpower. So when Koenigsberg falls, more people would flee than it would be if Germany had weaker laws.
After all, manpower only represents a military reserve population, not the population of a province. And that military reserve will not stay in an occupied province.
That is as far as your manpower goes.
But loosing a province will damage your manpower growth rate - as it did in all previous games. So if Berlin falls, you will no longer have its population contribute to the next month manpower growth. But the manpower you have gathered until then will not fall if a city falls.
It all makes sense on a level. Yeah, sure, you can argue that not all army ready (not all, but the ones you would get by the law anyway) manpower would retreat, but that is amount of detail and nitpick I think is not realistic for this game. Take it like I have said - army ready population retreats, civilians, factories, resources stay. After all, as Germany you have, what, 3 million manpower while the population is 60+ million? I don't think that a fall of a city would take that away from your total manpower, at least not to a degree that would impact a game so much that it had to be in.
At least that is what I think. You can always mod it in for big cities if you want. There are already events like "Fall of XXXXX" for major cities so you can add a manpower hit in there.