Leaves Turning Yellow on Purple Robe Locust?

I noticed some of you writing about a purple robe locust (robinia ambigua) tree. A landscape company planted one in my yard about five years ago. For the past several years, the purple blooms show up in June and stay for about a week, then fall off and the leaves start turning yellow and dropping off, although most of the leaves stay green. But the yellow ones are smaller and litter my lawn all summer long. I have had three different "tree specialists" and each has suggested spraying, which has not seemed to phase the tree. Anyone have any ideas? Thanks

Hardiness Zone: 6a

By shw3 from UT