The problem was that not one case of mumps was found to be in an unvaxed person. Not one.
The first pass was that it was in all unvaxed kids (we are in the Chiropratic center of the world here in Eastern Iowa, so that is a reasonable first guess). However, they didn’t find a single case. They traced it to a 4 year old girl at a local preschool, but were not able to trace it back from there. Her family hadn’t traveled out of the area in that time, and had no unusual exposure.
So we had 8 kids with mumps, all in a population of under 50, all in a cluster around the preschool and elementary school in my town. All had current vaccinations. That is well beyond the expected failure rate of vaccines. It is (if I remember right) about three times more than the expected failure rate. And there were no cases of unvaxed people with the mumps in the area. Possible that the exposure was in a transit point (gas station, Walmart, etc), but again they didn’t find the source of the first little girl getting it. Even among the illegal alien population.
I grew up on a farm. We knew, studied, and lived vaccinations. Dad kept very careful records of sick animals, and if he saw a cluster like this it would be a cause for revisiting the vaccine. Or a worry that some other bug just got into the population.
That is a big red flag that some thing is wrong. Now one theory, that makes a bit of sense, is that there was a bad batch of vaccine. This would explain some of the cluster, but there were kids older than the index cases that got it, and not one of the older people (parents, grandparents, etc.)
Given that mumps can be spread before symptoms appear, the incubation time is usually 16 to 18 days (range: 12 to 25 days), and four year old girls aren't very likely to be able to describe a situation where they might have been exposed, the index case had plenty of chances to catch it. It could have been in a store, where she touched something that had been touched by a transient traveler who stopped in the store earlier.
On average, the mumps vaccine is 88% effective after two doses. However, it is only 78% effective after one dose. Children are not given a second dose until between ages 4 and 6. Thus, on average, 22% of the children at the day care would have been susceptible. But, probability being what it is, more or fewer children could have caught the disease without that being any indicator of some alteration in the virus.
According to the CDC, there was an outbreak in Iowa in 2015-16, in which over a hundred college students came down with mumps. Perhaps one of those students stopped in your town before becoming symptomatic.
It would be interesting to see a phylogenetic analysis of the virus from the outbreak in your town and the college outbreak. Actually, those analyses can give a very detailed picture of the spread of the virus between communities, even if the actual person who transmitted the virus cannot be located.