Hi Trev!
This is not a bug and can be easily solved.
The animation looks that way due to the fact that when the change is really quick, like in the case of a quick passing of the cursor, the two functions (mouseIn / mouseOut) can not be resolved on time.
You can simply solve this issue by using a promise that will execute the second part of the function only after the first part is resolved.
Hope it helps!
Best of luck.
Doron. 