Your Japanese Lilac may be suffering from Bacterial Lilac Blight, based on the clear illustration & additional description, and your Lilac may be saveable!
Some Lilacs survive Lilac Blight, especially with care. Blight can get into a Lilac through open abrasions or from insects etc, & moves through the tissues in the plant, and progess can be very rapid. Sometimes Blight will take an entire Lilac, other times several branches. Insects can very quickly spread it from branch to branch, and from plant to plant, so control of insects that can spread it is very important.
To reduce occurrence & spread of Lilac Blight includes: prompt removal of all affected branches & leaves and other debris, keeping the leaves dryer when watering, and thorough removal of insects which may spread it. One insect can quickly spread Blight to several branches.
When trimming off affected branches, trimming back 60-100 cm from where there's moist tissue can help if Blight hasn't already spread through the branch; typically, an affected branch may entirely fail. Importantly, when trimming back or removing entire branches, dip the pruner in a 1/10 bleach solution before and after Each prune, and Immediately seal Each prune! And avoid any trimming off if it's raining or rain is imminent, because rain could spread it.
Removing the mulch from around the Lilac would be a good idea, because mulch retains moisture, and because mulch can attract & conceal insects which spread Blight.
Although some suggest that there isn't any treatment for Blight other than carefully trimming away affected branches & Immediately sealing them before it spreads to healthy tissue, and that the entire Lilac will fail, Some have had good results using copper eradicants: so perhaps check what may be available in your area, as it could be important & helpful!
Keeping watch for any newly affected leaves or branches and immediately carefully removing them is very important because Blight can progess so quickly. Hopefully your Lilac can be saved!