Southern Magnolia in Maryland?

Huh. It should do fine there.

It's totally normal for it to lose the older leaves (2 to 3 year old leaves). Spring is when you see most changing to yellow, and eventually brown, and dropping off. I hand-groom mine in mid to late spring to get most of that off and out, as it does detract from the look. But if it flowers appropriately in June(ish), and puts out new growth each year, it could be simply normal shedding of older leaves.

If the growing tips are turning brown, that's a horse of a different color, and I'd say something is wrong. Is it in a wet spot? They don't tolerate wet feet, and a soggy place, or one that has standing water for a few days after heavy rains would cause it to not thrive.

I would not prune it, as they're slow growing, and don't respond well to pruning. If you still think something's amiss, then get an arborist out there to assess it. You can't just throw "remedies" at it when you don't know what's wrong, and some things you do might only make things worse.

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Magnolias drop leaves all year (which is why their nickname is "dirty tree"). They don't need pruning. They should do just fine in Maryland.