No, gravel or other coarse stuff in the bottom won't help drainage. That's a cherished, but erroneous, belief.

When you planted out - what was the condition of each root ball?

Jacaranda often has a spreading and shallow root system. If, when you planted, you used a broad and fairly shallow 'bowl' shape and planted out on top of a gentle mound of good soil mixed with aged compost and backfilled with a similar mix, then water can drain away. The compost helps to retain moisture for the plant, keeps the area loose enough for young foraging roots, and lets vital air through as well.

As the plant has not been in the ground for long enough to really settle you could unearth it to see what is happening in the root zone.

Gravel in the bottom of the hole doesn't help. However, sharp grit - about 5mm diameter - mixed through the soil/compost can encourage roots to spread out. Not 'heaps'; half to one bucket to about ten buckets of your soil mix.

The other possible is that the Jacaranda is starting to wind down for winter. Some of them drop their leaves over winter and early spring prior to flowering.

Also have a quick think back over your temperature range recently: any chilly mornings that might have given a touch of frost damage? They're not fond of frost - particularly when young.