Yep. I am in the middle of a shell off restoration and I found exactly what you describe as a manufacturing defect. They do not seal the lap seam- except to slather on vulkem on the inside AFTER its put together.
Water finds a way....
The awning rail becomes a gutter. Water flows like a river down that seam. It was making its way in SOMEWHERE and I couldnt find it. First I drilled out 1 window and resealed.... then I did another... then I drilled out the top of that seam and injected sealant before re-riveting... then I did the middle of the seam.... eventually I had drilled out and re-sealed the entire seam and now I have no leaks there.
Now when I see pictures of damaged flooring in peoples Airstreams, I find this to be a common rot point in the same places on the floors....