There is a reason why I always clamped the seat stays at the brake bridge when spreading a frame from 120 to 126mm.
Yes, the seat stays do need to bend slightly when spreading a frame.
The Waterford way mentioned where they replace the bridge is interesting but it just transfers the stress up to the top.
Hmmm.
If the frame was out in an unheated space during the Polar Vortex freeze.... That could be a contributing factor.
Amtrak stopped running in/out of Chicago for a reason during the worst of it.
Very cold metal gets very brittle.