I planted 3 little gem magnolias a few years ago. The first two years they looked like heck. And they do go through a molt, although that usually happens in later winter.

It's probably just transplant stress. Just make sure they have plenty of water, but don't drown them. I'd lay off the fertilizer until late winter, early spring.

None of the bacterial and fungal diseases of Magnolias are considered serious and no remedies are recommended, other than collecting and disposing of the fallen leaves to prevent reinfection. Overwatering can promote the growth of these pathogens.
Planting near the foundation of the structure may subject the tree to lime that leaches from concrete and mortar. Magnolias are somewhat intolerant of alkaline soil, which prevents the uptake of necessary micronutrients, and may develop chlorosis (yellowing leaves with green veins). Add elemental Sulphur (4 Tbls/sq. ft.) to lower the pH.
Be sure that any mulch placed around the tree is dry and fresh smelling, not "sour", which harbors toxins. Keep mulch a few inches away from the trunk and limit the thickness to a few inches to allow oxygen to reach the tree roots.
Good luck!
Rb