I seeing more and more YJs but they are not a problem for the bees, yet.
This tells me there is a nest of them close to my house. I keep an eye out for them and I will destroy them if I find them. You may have a large nest of them that is in a warm location and it is not dying off and having to start over again like they normally do.
At the bee college, which is next to the entomology building, they have YJ nests that are monsters because they were in a large underground void and did not die every year.
Jim