Brown spots on leaves
There are many reason why such symptoms can occur on the lemon tree. You provided some information that this is a new tree.
These are some of the plausible and common reason why such symptoms, which often are result of some kind of environmental and/or physical stress, are noticed:
1. inconsistent care - While in pot, the soil/media should be moist. These problem can arise if the root ball got to dry at some point (ie. the tree don't like too wet and period of too dry.
2. Damaged roots - If tree was transplanted into a larger pot or the ground, it would undergo something called "transplant shock", where it will take some time to adapt. In the meantime, those "bruised and battered" roots may not function at optimal, hence the symptom appearance.
3. Too much care - a common mistake made by plant owners is to feed the plant when they first enter the family. Here have some fertilizer - did you know fertilizer is essential a "salt". Too much fertilizer can cause such kinds of burns on a plant.
Check out this online resource to help with getting a citrus plant growing well: https://aggie-horticulture.tamu.edu/fruit-nut/fact-sheets/citrus/