Water Accumulation in Fiat Egea Spare Tire Compartment

I purchased a 2018 model Fiat Egea Easy package vehicle as a second-hand vehicle six months ago. I recently had its major maintenance done just a week ago. While the car was parked in front of my house, noticed a continuous water leak under the rear trunk. Initially, I thought it might be an oil leak from the differential, but upon closer inspection, it turned out to be water. I checked the water level, and there was no loss. The dripping spot is, in fact, beneath the trunk. Next, I opened the trunk to see if something was placed under the spare tire area, and to my surprise, it was filled with water and had developed mold. Since I purchased the car, had even had the interior cleaned thoroughly, wondering about a persistent odor, but it never went away. Apparently, the spare tire compartment has developed mold and started to decay. Why is rainwater accumulating here in the first place?? what should I do??