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Pictured is the bottom side of a Mazar leaf. I cannot see an egg in the thing... but it is directly on a vein. And whether through or not i dont know, but there is a dimple on the top side of the leaf at the same spot. Almost like it was pierced but i cant say. This is not the only leaf on the plant like that either.
Looks like some type of gall. They are usually from an insect or mite, but can come from fungus or virus. The piercing you describe from above is the initial injury, probably a wasp.The gall forms around the developing life form and eventually it will exit the leaf.
Galls are on lots of different plants if you look for them, on herbaceous and hardwood plants. With leaves it's mostly cosmetic damage, some galls will kill off branches, pin oaks are a good example of that. They get hammered with galls and there is no real cure, can only prune it out.
Spent a few years doing insect and disease control for a larger company on trees and shrubs, we had everything chemically available, and the best that could be done was killing the fungus or insect while it was attacking the plant from the outside. Once you see the gall it's too late, but again, it was just something people got excited about but only cosmetic unless it was on the branches. Leaves come and go, no big deal.