This is almost certainly the answer. The game now only displays changes in your income at the end of the month, though changes you made in the previous month do take effect. In your case, this means that you won't pay the upkeep for the fleet you disbanded, even though the tooltip says you will.Did you unpause to let the game recalculate?
NSC itself adds fortresses back too, no need for another mod.If you want to make proper defence stations that can stand up to fleets later on, you'll be needing the NSC mod plus another that adds in bigger defence stations (the old fortresses etc are back!). Both of these massively improve Starbases over all and make them viable as strongpoints, even later in the game.
Also, you can see the maintenance cost of a station in the ship designer, the same as you would a ship.
NSC itself adds fortresses back too, no need for another mod.
You can see the upkeep in the ship designer when you design the platform.The description of the platforms also make no mention of the upkeep.