Its been pointed out in the comments that serum magnesium is not a good biomarker for tissue magnesium.

As such high serum magnesium may result from impaired magnesium transport to tissues, perhaps?

So high and low Mg might indicate a Mg distribution/regulation problem, low serum Mg would starve the transport system to tissues, leaving tissue deficiency, whilst high serum Mg could indicate a blockage in the uptake system, also leading to tissue deficiency.

Is Mg transport energy related? Features of diabetes in the brain and impaired brain metabolism is known in dementia and Alzheimers, and diabetes is a risk factor.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4455825/

Serum Mg is largely controlled by kidney excretion.

Mg is known to be difficult to deliver to the brain via supplemental routes and different Mg forms may influence this.

So, if the transport barrier to brain uptake is increased in dementia then the serum level might not tell us much, and it could be that it is showing there is an issue getting it into the brain.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6678825/

Asside from kidney dysfunction in regulating Mg it could be that bone decay is releasing Mg. It's known that as we age the bones release elements like lead that have been absorbed throughout life, so perhaps this is an added source of high Mg.