Sleep,

Are you growing it in Florida?

If I recall correctly you live in South FL? Not a lychee expert but I would venture a guess it has to do with your soil moisture...I live in Central FL and our weather has been very erratic here lately...first its BONE dry for weeks on end without rain...then one day heavy rain then bone dry again. I'm guessing the inconsistent dry and wet ground that "might" possibly attributing to the fruit splitting? I've heard that some mangoes like Nam doc mai has the same problems with inconsistent soil moisture. Aain, just a guess?

I just got over 2" of rain in less than 24 hours, I'm hoping this strange Spring we're having isn't going to cause problems with my Mangoes...Good luck!

Yes, I am in S. Florida. The weather has been very inconsistent, and yesterday it rained quite a bit.

That tree is also in an area of the yard that is pretty dry. A majority of my mangoes are planted there for that particular reason. Maybe good for mangoes but bad for lychee..

Pic of the tree today....

sleep,
There is not much you can do to solve the problem now. When the fruit is young and there is not enough water the cells that allow for the expansion of the shell harden and will not continue to grow while the fruit on the inside continues to grow. That causes the split.