Subaru’s have been blowing their head gaskets since day one. It’s VERY common for them to have this problem.
Idling at the shop might not expose this problem as there is very little cylinder pressure during idle…
Perhaps Subaru owners should have the heads re-torqued every 40,000 miles, something like that, in order to reduce this problem. Don’t expect Subaru to ever recommend this, as this would be admitting they have a design problem. I feel the expansion and contraction of the “pancake” aluminum engine slowly stretches the head bolts and their clamping force is reduced until the gasket blows…The ones I have checked have ALL been well below torque spec. ANY overheating episode, for whatever reason, will loosen the head bolts and lead to gasket failure. It’s just the nature of the beast.