When using grid, the x and y returned by onStop are incorrect. lastX and lastY however seem to be correct. I can't find a explanation for the values onStop returns for x and y.
I've created an example showing the behavior:

https://codesandbox.io/s/texthighlights-wtrzt

Please have a look at the console output when dragging the object around. I would have expected x and y to change based on the grid of [50,50] but this is not the case.

React-draggable is also used by react-rnd - they also have an issue open for this bug (bokuweb/react-rnd#453)