Having the same issue on the latest version, not sure if its supposed to be an intentional way to make manpower scarce for the Metro setting and slow down map-painting or if its my garrisons and will stop once I core everything OR if its a bug.

EDIT: In case anyone else reads this or the OP does, I tested coring ALL of my land to see if it changes. Manpower still solidly depleting at around 40 guys a month with no explanation or input from my end. Means its either a fatal bug with some event tied to manpower in the beginning of the game that continues on, OR its a design decision by the dev because like OP also said, older versions didn't have this, your manpower just grew.

A 50k population map w zero growth on top of NEGATIVE manpower growth is a bit much IMO. Maybe in the future when theres more content it'd make sense, but without any mechanics to respond to the dipping manpower its really just begging for the player to open up console and cheat a few thousand guys in.