I've not heard of this type of failure on the Brompton crank arms, and looking closely at the threaded holes where the pedal spindles screw in, there is no apparent difference in the amount of material or thickness between the left and right crank arms. Whether the pedal is folding or not shouldn't affect the load or stress on the threaded hole and surrounding material.

I'm guessing the cause is a manufacturing defect rather than a design flaw. Can you post close-up photos of the failure?

EDIT - Reading the OP, I thought it was the arm that broke at the threaded hole, not the pedal. If it was the folding pedal that failed, I can see it being a design flaw.