It probably happens too fast to be noticed at normal speed, but Luke has a pretty good grip on his saber.
And now I have the mental image of his hand grasping the hilt long after it falls.
Don’t forget that it was almost canonized in TFA that that hand would still be grasping the saber thirty years later (and that it also made its way into space somehow).
https://www.youtube.com/watch?v=rolEdZ3c2Fk&list=PLYn2a0e2FVIKni_wHXR5CJYBnnnYH7mIB&index=121&t=0s