Thank you for contacting Ask Extension. Without seeing your lilacs or analyzing your soil, it's difficult to know what the problem is. But this year, with the cold temperatures and then heat, and lots of rain and then none has not helped any lilacs in the area. The brown spots are usually indicative of a fungal infection, due to the above weather issues. Turning brown is also indicative of the shrub getting ready for fall, because of the cold temperatures. 

Fungicides are not effective at this time on plants already infected. Next year, a fungicide should be applied in the spring when the leaves first emerge. Do not plant lilacs closely to other shrubs, thus allowing airflow around them. Water from below and make sure the roots receive water during hot dry droughted periods.

The fertilizer that you used should have been alright. But in the future, do not fertilize any shrubs or trees between July and November due to the fact that you want the plants to begin the process of dormancy. Fertilizing at this time may encourage a
flush of late season growth subject to winter injury, and retard the normal onset of dormancy in the fall, greatly increasing the risk of winter injury.

I have included several publications for your reference:

https://extension.illinois.edu/blogs/good-growing/2021-08-05-whats-wrong-my-lilac-summer-foliar-diseases-lilac

https://monroe.cce.cornell.edu/resources/growing-healthy-lilacs#:~:text=Never%20fertilize%20lilacs%20between%20July%201st%20and%20Thanksgiving.,fall%2C%20greatly%20increasing%20the%20risk%20of%20winter%20injury.

https://extension.umn.edu/trees-and-shrubs/lilacs