That's normal, they loose leaves all year long. In the Spring more, as long as it is setting new buds, I wouldn't worry. Sounds like you are doing all of the right things.
Good luck
jolana

It's an Iron Deficiency..My neighbor had the same problem and I suggested Iron and it stopped...Jeanne

I've heard of folks using Ironite on their mags. Last year was the first year I used an organic blend acid fertilizer and even with the drought, I had a ton of flowers this year. Not supposed to fertilize until after it's done flowering, which may already be here for you.

They do lose leaves in the spring. Right now it's hard to stay on top of clean up. (I usually rake them and mulch / mow above the roots and let the mulch sit)

Watering is critical for newly planted trees, expecially when it starts getting hot. A tree dissipates most of its water through its leaves. If it loses its leaves, it won't need as much water. Feel the soil to wee if the soil is wet or dry and water accordingly. If your tree is puting on new leaves, most likely it will be ok. Pay attention to your watering.

This is from another forum here at GW

Here is a link that might be useful: Magnolias

My magnolia is just over a year old, and made is through the San Antonio ICE this year, it is about 6 feet tall, but has not bloomed yet, I am also using muracid, I think that is how it is spelled, lots of peat moss, and iron shavings. It is very green, growing in full sun. Does it take a few years for them to start blooming? Also, are there suggestions on how to get the truck larger in diameter?