If the 2nd lamb was deep down inside then it might not have been easy to feel... I had a set of triplets this year, the first 2 came out fine, the third took a while to come up into the birth canal. If she wasn’t scanned for 3 I probably would have missed it. This could well have been the case if the ewe wasn’t dilated fully when the vet arrived. But yes, I would ask if the vet didn’t check for the second lamb then why. It is easy to blame the vet though and sometimes it’s not their fault.
I remember a few years ago we had a cow calve a healthy calf... penned her up, calf sucked, all good. Came back 3 or 4 hours later to find a dead twin in the pen. Was it anyone’s fault? No not really.