Looks like it’s Zone 3-7. It might need more of a dormant period than your Zone 8 can give it?
https://www.thespruce.com/japanese-lilac-tree-care-and-growing-guide-4589076
Greetings from west Michigan where we have had about 3” of rain in 2 days and it is still coming down like crazy, guessing 4” by the time this storm passes through. Fortunately our house and garden are on a high hill, flooding never an issue, although our creek at the bottom of the hill most likely is a river by now. If it stops raining today I may take the atv down tomorrow to check it out. Garden is doing great, lots os small green tomatoes broccoli doing well, no heads yet, some lettuce ready to pick and I would love to have a salad this weekend but probably won’t be able to pick til tomorrow morning. I’ve been making rose water from a certain rose bush from our old house in Chicage. Dad swiped it from their old rental shack (yes shack) when they bought that house. It’s a damask rose called Amelia and he cut off a chunk (the root ball was like a block of wood) in the late 80’s. They bought that house in 1946 so the bush is at least that old. The damask rose was heavily used for it’s strong perfume. Registered in the 1800’s. Truly the most wonderful smelling rose ever.