This is a bug. I have a PC and I use it the same way that OP is saying. I use dip to white as a “flash from white” type of effect by moving the transition to 50% at the start. I do the same with dip to black. This works 100% great on my PC at home. On my Macbook for work is a different story. It does just as OP is saying, and no matter where you adjust the start or end of the transition to it always plays as if 0% – 100%. I sorta feel bad for the OP as everyone is treating him like he’s unintelligent for asking this question when it isn’t working as intended as some have suggested. Of course you can do the transition old school, but it would be nice if this bug was fixed to save time.