0.1% is almost zero but I’ve noticed that in older safari that some animations don’t work with zero but do if you give it almost zero (or do it in reverse). It may be that the 0.1% was an old bug fix. 0% is the start of the animation and 0.1% is virtually the same thing.
The shorthand rule 0%,100% just sets the start and end properties to the same. You probably have a 50% rule in there doing something different so you get animation to 50% and then back to start.