All of our landscape plants have been deeply stressed over the last couple of seasons by intense heat and prolonged periods of of 'concerning' to 'severe drought' -lack of rain.
Young, new plantings have limited root systems to deal with this well, and some leaf scorch can occur. (Supplemental watering when we get less than an inch of rain a week is important, esp. for the first couple of years a new tree or shrub is in the ground: https://extension.umd.edu/resource/watering-trees-and-shrubs/)
Like us, when under stress we can be more prone to disease.
That said, if most of the leaves have large areas of green tissue like the ones in your photo, they are likely fine. Lilacs and some other landscape plants can be prone to some leaf spot diseases, but they are primarily cosmetic and do not harm the shrub overall in the long run. (i.e. It's normal and o.k. for plants in a home garden to not have perfect leaves).
Here is more about leaf spot disease: 
https://extension.umd.edu/resource/fungal-leaf-spots-shrubs/

And our page on Lilacs: 
https://extension.umd.edu/resource/lilac-identify-and-manage-problems/


Christine