OK...back to the topic.

As I said before....I think the chances of the frame welds breaking without serious trauma is pretty unlikely. Typical American and Japanese cars use the 4*sqrtGMT. That is the weld button diameter must exceed 4 times the square root of the governing metal thickness. Most German applications use 5sqrtGMT formula. Given the gauge of the frame material and the formula to figure weld size, there are some pretty beefy welds under there.

That being said.... since this was a problem since the car was new, there's 2 areas I would start looking. First, since it's a Roadster, I'd check the underbody brace for location and tightness. Second, I'd take some rudimentary measurements to make sure something didn't happen during shipping. There could have been issues where the car was tied down by the frame or shipping loop in the back of the car.

You never mentioned if this was an early build date or not. If it was one of the first few hundred off the line, there's always the possibility that there were welding issues with the stepper schedule, cap geometry, weld-through sealers, etc. but I'm going to assume not for the moment.