OP Deleting Question Immediately After Answered? [duplicate]

I recently answered a question, Classify stationary points of $f(x,y) =8x^3-3x^4+48xy-12y^2$, and it was deleted immediately after answering.

I found previous posts like this, but this is very odd and seems like it should not be allowed.

Why is an OP able to delete something as soon as it is answered?

I realize it can not be deleted if it has upvotes, but it is deleted immediately after answering - where no one even has the opportunity.