Some forms of magnesium trigger more stool my movement than others. And there are many forms.
I had opposite problem with it, tried ti find what won't cause it since I have a problem with it too behind with. Also sour kraut triggers me almost always and instant toilet break.
You mentioned elsewhere that milk of magnesia worked, so is it possible that you received a bunk product? I know this is a common issue with supplements and there are some neutral third-party test labs for this.