I have a Magnolia grandiflora (Southern Magnolia) that did a similar disservice. The leaves that normally were a healthy, medium, shiny green, curled and went brown just this past July. I have no clue as to why. However, I suspect the the unusual amount of rainfall we have had this summer may be the cause.

My soil is clay, that I mix with compost and sand to create a better growing medium. Given the amount of water we had on almost a daily basis, I tend to doubt the ability of a clay base soil to drain properly.

In desperation I redirected the water away from the tree with a small furrow. The tree is now responding well and new leaves have sprouted.

I am not sure if this helps or not. Given that your situation involves a different type of Magnolia and a different soil, there may be a very different cause for the dieing off of the leaves.