I've had this same issue since about 6 months into purchasing a NEW 2018. Had it into Dealer twice already to have fixed. Leaking continues do show up during heavy downpours and recently I've noticed that if I make a right turn, the water simply pours in and soaks the interior. I am giving the dealership one more chance to fix the issue, once and for all. I'm seriously concerned that the water is going to wreck the electronics in the rear hatchback (wiper motor) and/or cause corrosion within the rear hatch and spare tire area. I'm sure Mold will be forming soon as well.
Dealership stated that it was the 3rd Brake Light assembly leaking, AGAIN. They've already changed that entire assembly once. They stated that they were just putting sealant around it. My question is this, what happens when that bulb blows and I have to remove the assembly to replace the bulb? What a bunch of idiots!!! I'm seriously considering trading it in now.