Certified GKH Gardening Expert
They are very low feeders. They should be fed an "acid-lovers" mix. Citrus fertilizer will do fine here, as will blueberry fertilizers.
Any instances of apparent nutrient deficiency should be looked into as an infection, or pH issue first. If you can rule out those issues, then you can begin to diagnose deficiencies, since they are not common in the tree.
This article will give you more information on their care: https://www.gardeningknowhow.com/ornamental/trees/ceylon-cinnamon/ceylon-cinnamon-care.htm