spark plugs burnt out in 12,000 miles?

I got a dual-fuel car in the workshop. Hard to start, misfiring on gasoline or propane. So check ignition, and spark plugs gaps are 1.6 to

1.9 mm. These were special LPG spark plugs with gap only 1 mm. Customer said he got a "full service" that changed the plugs only 12,000 miles ago. What could cause such rapid decay? I wonder if the other workshop has cheated them, not replacing those plugs.