When you set a sprite’s animation, if it changes from one animation to another then the animation is restarted from the first frame. However, if it’s the same animation, it continues playing it.
The problem is when you press 2 keys at once, it sets the animation in first keypress event, and then resets the animation in the second keypress event. Because the animation changes for the second keypress, it resets it to the first frame. And it does that repeatedly, giving the appearance of the animation pausing.