Used FX 8 b100.
To reproduce run the attached app.
Click the splitter, move it by say 1 or 2 centimeters to the right, do not release the mouse but wait half a second then move again the splitter (in the same direction or the other one, it doesn't matter).
You see that on the first move the splitter follows the cursor as with every further move it doesn't. When you release cursor then the splitter position is resumed.
The expectation is that the splitter follows the cursor all the time until it is released (as does FX 2.2.40).