When it cools down, it returns to its previous state, even getting harder. Why? Because we have removed some more of that moisture.
Sometimes we even use it when the fondant is freshly opened, because we are not able to knead it, because it is hard and difficult to handle. But we are also subtracting moisture.
That is why it is preferable to avoid it as much as possible and try to use other methods and if you use it, try not to abuse it.
If, for example, you have a hard time kneading it, in this post you can read some tips to warm your hands instead of fondant and you will be able to add heat to it. Thanks to that heat, you will get it to soften, which in the end is what the microwave does.