Strange that it's not doing it more consistently. It has to be the mag release not making positive enough contact with the mag.
I'd remove the grips, strip the magazine so you can see good and see if the ledge on the mag release is making good contact.
Since two mags are acting up, I assume there's something not right with the release itself.
Funny the gunsmith couldn't get it to do that and even if he couldn't he could check out the release or replace it just to be safe.
In Memory of Paul "Dietrich" Stines.
Dad: Say something nice to your cousin Shirley
Dietrich: For a fat girl you sure don't sweat much.
Cue sound of Head slap.
RIP Muggsy & TMan
"If you are a warrior legally authorized to carry a weapon and you step outside without that weapon, then you become a sheep, pretending that JOCKO will not come today."