How was this seam ever sealed at the factory? How could I seal it now?

I replaced the panel in between the rear window and the trunk around 2012 and remember thinking at the time, "how the hell will that ever seal." Fast forward to getting the car painted in 2023 and I had forgotten about that and now it leaks. You can shine a light and see through in some parts which makes sense since it's just two steel panels pushed up against each other with a few spot welds.

I found some threads where people said this is just a problem spot on these cars which I can believe for sure. I am wondering if anyone has had luck fixing it. I was thinking it might take sanding off the paint and putting in some filler or something, but I don't want to lose the seam.