Q.Why Has The Top Of The Central Leader In My 17 Year Old Kousa Dogwood Died?
I planted this tree 15 years ago (it was about 5 feet tall then) in a south-facing alcove of the house. It has appeared to thrive in its location since then, but this Spring I noticed that the top of the central leader is bare. The rest of the tree appears healthy…plenty of blooms this Spring. My irrigation system keeps it appropriately watered. The tree is now about 18 feet wide and perhaps 20 feet tall, partially shaded by a tall River Birch in the morning, full sun in early to mid afternoon, and shade from the west two-story wing of house in mid to late afternoon.

Certified GKH Gardening Expert
Dieback always warrants an investigation. Here are some articles to help you pinpoint the problem:
https://www.gardeningknowhow.com/ornamental/trees/dogwood/common-dogwood-problems.htm
https://www.gardeningknowhow.com/ornamental/trees/dogwood/dogwood-leaf-drop.htm
https://www.gardeningknowhow.com/ornamental/trees/dogwood/dogwood-blight-control.htm