Well Mg is the 8th most abundant element on the planet (and probably in the universe). So the problem has nothing to do with shortages of it (13% of the Earth, and approx 2% of the Earths crust is Mg in one form or another). If you have ever heard of dolomite, that contains Mg, it is CaMg(CO3)2. Many people have it as a driveway, its used in terrazzo concrete and tons of other things.

So its not a shortage. What I can tell you that 85% of global Mg output is from China. Back in 2021, China was struggling with high coal and electricity prices (both which are needed to refine Mg to 99% after its mined out of the ground and floated). They shut down 50% of the Mg refineries, and then the started those ones back up 2 months later, but only at 40% capacity, until 2022. What we are probably seeing now is that "hiccup" in production rippling through. The other thing I can say is that plants are designed to work to capacity. They cannot usually "just drop the output" It just doesn't work like that, they dont have a "volume knob" to reduce the output to 40%, and when they are running out of design bandwidth, they are HORRIBLY inefficient and wasteful. They would have been starting, run at full capacity, and shut down again, and it makes me shudder thinking about how bad this is. (plants are designed to run with, oh maybe 2 planned, full shutdowns a year!)

China's supply to the world (the 85%) is about 0.84 million metric tons. So, in my opinion, dropping to (probably less than) half that for ~ 4 months is a big deal to supply, and we are still feeling the effects although I suspect we are getting past it now.