The left sear (assuming a non-selective trigger) may be too light an engagement now after adjustment and it's just occasionally falling from the recoil of the right bbl being fired. Maybe it's not engageing all the way when opening to cock it.
The inertia block only holds the trigger from movement during it's recoil induced engagment.
The sear itself is still free to move in most guns. If the engagement is light or faulty,,it can still slip off the sear notch of the hammer and let it fall.
I'd return it to the well known gunsmith to make it right. He should know how if he's all that good.
