We used to have a lot of laying hens for about 10 years and seeing this kind of shell-less egg was fairly normal. Not super common but nothing to get alarmed over. The chicken that laid this kind of egg would do it once and likely never again. Hubby wonders if their feed perhaps does not have enough oyster shell in it. Chickens need oyster shell or gravel or something in their feed to help their gizzards grind down their feed or else they will starve to death, even though they're eating.
The first year we had meat birds, they did wonderfully, eating the gravel on the ground. The next year, we put 50 birds in the same spot and 46 died. We knew nothing about raising birds and had to learn the hard way that they had eaten all the gravel the year before and we didn't have anything similar in the feed the second year. We didn't even know that they needed such a thing. The only thing I knew about chicken gizzards was that my mom ate them and they looked gross!
Having said all that, that still might not be the problem. Just wanted to pass the info. on. The feedmill salesman might know more than a vet. but of course, he might also want to sell you his product! These eggs meant happy times for our dogs and cats!