Hey Label, as Omar said, this happens from time to time.
But to shed a bit more light on it, from my experiences in the past, there are certain ‘edge cases’ where the solver for the boolean operations gets confused / messes up. And instead of for example cutting a hole into the mesh you wanted it to, it does an ‘inverse operation’ where it deletes everything but the cookie cutter shape.
I used to solve this via either changing some settings, as Omar suggested, but found that sometimes this ‘edgle case’ can be avoided by simply shifting your cookie cutter mesh (in your linked post, the star) by a bit.
That is sometimes enough to get it working correctly, even if it means its not EXACTLY in the correct spot where you wanted it.
(Mind you I worked with this back in 2.79 so maybe its now caused by something different, but give that a go and let me know).
