Unless something dramatic happened - like the watch fell to a concrete floor from 6 feet (or something like that) - Zenith should fix this for free. Inform them that a steel chronograph part like that should never break simply from normal use. I once had ...
Something wasn’t right regarding the clearance, and over time, with usage, it’s suffered material fatigue. There’ll be a fair bit of material debris in there, and it’ll accelerate wear on pivots, etc – so don’t even be tempted to continue wearing it. ...