On a character, I set the priority of a custom animation to the lowest priority (core), and play the animation as follows:
animationTrack.Priority = Enum.AnimationPriority.Core animationTrack:Play()
So what I expect to happen is if I move my character with WASD that it will interrupt this animation and show the walking animation (since the movement animation has a higher priority).
When I test the game in Studio using Play, then this is exactly what happens. When I move my character, the custom animation is replaced by the walking animation.
Yet, when I test this using Test: Start (or if I test it in a live game), then my custom animation keeps playing and is not interrupted by the walking animation.
So this looks like a bug. It is easy to replicate by loading an animation using the code above, and then moving your character with WASD.