Left bbl,,check the trigger shoe itself inside the guard. Many are set well to the rear and the bottom tip or backside of it may actually contact the inside of the TG bow when the gun opened and the lock is cocked.
The trigger then moves forward a bit as the sear falls into it's cocked position and allows the trigger to also drop a bit.
But if the cocking motion (dropping the bbls) is less than complete (babied),,the sear won't be allowed to drop into place as the trigger is pushed against the bow.
Just enough resistance . That leaves the sear to hammer engagement 'on the edge'. A light touch to the trigger sets it off.
If it is the problem you can often see a light burnished mark, sometimes not very obvious at first glance, where the trigger touches the inside of the guard.
A real oddity is the shooter that pokes their middle finger of the right hand inside the guard as the thumb on the RH is used to open the top lever.
That middle finger can go right up against the rear trigger/left bbl or press up on the usually long bar that is the forward portion of the rear trigger.
Either can hold the trigger back slightly which effects how the sear drops into the hammer notch.
I'd imagine the front trigger could be effected the same way, but both would be a real rarity in handling/function. Only something you yourself could likely diagnose and 'fix'.
It can only make the above problem appear if the gun has a Non-Auto Safe (or an Auto-Safe that is very sloppy in it's engagement with the triggers for the SAFE condition).
It's more likely worn cocking linkage part(s) along with less than complete opening cycle of the bbl's.
Worn sear engagements will generally show the same engagement features with every pull wether very light, heavy, gritty, whatever.