I accidentally poisoned myself with B6 from Magnesium tablets over the course of a year: https://davids.town/vitamin-b6-overdose (my levels were 38x the healthy range).
tl;dr: Always check your Magnesium for what else the tablet includes. If it contains Pyridoxine hydrochloride (i.e. Vitamin B6) or another Pyridoxine compound, find one that doesn't. Since then, Swisse is the only brand I've found that consistently sells "pure" Magnesium tablets here in Australia.