Persistent Hypomagnesia

I have a patient who has a history of unexplained persistent hypomagnesia. It’s so bad that he has had TdP and runs of VT and has an ICD. He had had testing for Fanconi syndrome and Gitelman syndrome which were negative.

However, he is currently taking 1000 mg magnesium glycinate QID and still can’t get a mag level greater than 1.6.

Has anyone encountered this?