No worries, seat strut cracks do break from time to time, being the one closest to the engine most likely the seller did not notice, or it fail while driving
I’ve had a few cracked on me several times from fatigue. This doesn’t mean you got a bad chassis.
Very important to have the right angle and correct bend on the strut before installation, otherwise it will fail again, this means if it doesn’t touch the seat once you install it at the cassettes then it’s probably not at the right angle.
It needs to be flushed with the seat and have very minimum pressure to it, almost like spring loaded rod.
You may get away by re welding it and be ok for a long time, but may be a good idea to always have some spare ones ( you just never know when one of them is gonna fail during a race weekend)
I hope we’re talking about the chrome seat struts and not the actual chassis tabs? Good luck to you and have fun. Fast Freddy.