Thick film resistors failure mode
We can only assume you used safe margins of 50% for power dissipation and working voltage.
With 240Vac and 1206 components that means using 100Vac per part instead of 200Vw and using 0.125W max instead of 0.25W.
I would hope you have at least a string of 4 Resistors with Transorb or MOV protection.
Being large parts they must be placed in parallel to board edge breaks for depanelization de-stress.