The reason that form isn’t valid is because for many conditionals of the form “if A then B” it is the case that the set of things which have property B is larger than things which are A ... this is a little non rigorous, but I think you’ll get my point. For instance if A is a car then A has wheels, does not imply that if A is not a car then it does not have wheels, since many things which are not cars have wheels. But the fact that this is an invalid deduction doesn’t mean there won’t be certain cases where the conclusion happens to be true, like in the case of the equilateral triangle. Where it must have the same angles to be an equilateral. But just because it happens to be true doesn’t mean the deduction is valid.