My own experience mirrors Julian's -- for my original pins. I save an astonishing amount of old bike stuff "in case it is useful someday" and find it often is! Salvaged things like old derailleur B-stop and limit screws, shoe cleat retainers and screws, brake pad holders and such go a long ways toward creating a useful bin of touring and shop spares.
I've occasionally come across pins that seemed intended to bottom out in the shoes themselves rather than travel through to the bottom hole of the pad holder. These I considered yoo dodgy to use and set aside, choosing instead to reuse the ones appropriate for my needs.
You really don't want to have a pad sliding out while parked on a hill.
I'm on my mobile while clearing brush at my little cabin on the coast and can't see your photo too well in the bright sunshine, at the moment so I'll ask a question I'd know if I could see better: Is the closed end of the pad holder facing forward? Not intending to impugn your mechanic, but that could account in part for things loosening so quickly under braking.
All the best,
Dan.