Water retention?

Hello! I have been taking 400 mg of magnesium glycinate (from double wood) for well over a year now. Shortly after I started it, I noticed that I started packing on weight—seemingly impossibly fast—despite making no changes to my diet or exercise routine. The timing lined up enough that I even googled whether mag could cause weight gain, and everything I read suggested it does not. So I kind of forgot about it and just accepted that I have put on weight. Then yesterday I came across in the supplements sub several people claiming that mag glyc caused them to retain lots of water, between 5 and 20 pounds (!!). I would love to know if anyone here has had a similar issue, and, if so, what the mechanism might be. I do not want to stop the mag because it really helps with my sleep. Thanks!