As wood dries, it shrinks. The shortest distance between two points is a straight line. Arched surfaces shrink and flatten as they dry, which is what you have observed.
What was the humidity level in the summer when the body and its components were assembled? If the humidity level drops below that, you run the increasing risk of having components crack, same as is true for finished instruments.
I glue flat bridges to domed tops. I've done it that way for four decades with good results. Arguments can be made for both contouring the bottom of bridges to match top arches and for gluing un-contoured bridges. You chose which you think is better. Objectively substantiating your choice is more difficult.
In climates that see considerable variation in seasonal indoor humidity levels - typically colder climates that heat indoor air - if one is to make guitars year-round, humidity control is required. It doesn't have to be complicated or expensive, necessarily, but is required. It can be as simple as a walk-in closet with a de-humidifier, or, if you don't have a lot of materials to control, a large box heated with a light bulb. Floor model humidifiers will keep humidity levels up in the winter months. I don't think putting sponges inside an unfinished guitar surrounded by dry air will be very effective. All of the wooden parts will expand and contract in response to humidity changes.
My current shop is a two-car garage. I heat it in the winter with a small space heater and humidify it with a standard floor-model humidifier. The humidifier consumes about a bucket of water per day, less if I keep the temperature a little cooler. In the summer, I dehumidify using a standard floor-model de-humidifier. I can easily keep the humidity constant year-round at 40%. Parts of the year require neither humidifying nor dehumidifying.
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