What causes the leaves on some Crepe Myrtles to turn a yellowish color but rarely fall off?
What Causes the Leaves on Crepe Myrtles to Turn Yellow
Certified GKH Gardening Expert
This sounds like a nutrient deficiency, likely iron chlorosis. Here is more information on chlorosis: https://www.gardeningknowhow.com/plant-problems/environmental/leaf-chlorosis-and-iron.htm
As a precaution, I would fertilize the tree with a general fertilizer too.