Could it be that during the weld they had too much heat on the tube which weakened it for the long run since you said it’s around where the bottom bracket was attached?.
Might be human error because all stridas are hand made and not by machine I found out. On my bike i’ve been commuting daily with it for a year so far (12km a day) and everyday I stand on the pedals straight up to do a stretch near the end of my spin. So far I didn’t notice any fatigue or even a crack on my bottom tube yet. It’s a pretty beefy tube so it should withstand quite a bit. I’m 155-158lbs though so I doubt i’m putting enough stress on it.
Not that i’m saying you are heavy at all! 
and keep us updated on how the new tube is working out for you! stand on it and give it some stress to see how it’s working. Just no bunny hops and jumping 