Your photos are very helpful.  It does look like a leaf spot disease, most likely a fungus.  It may clear up without any chemical treatment.  It's important to avoid overhead watering and to clean up any leaf litter on the ground to avoid further spread.  Below are a couple of articles that provide more information on leaf spot diseases.  Good luck to you with your magnolia tree. If you decide you need to treat the tree in the future, you may want to call in an arborist. 
https://plantdiseasehandbook.tamu.edu/landscaping/trees/magnolia/
https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs