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That looks like mechanical damage. Something bumped or cut the leaf while it was still in the bud or just starting to expand.
It might be a bug, or it could be something or someone hit it or nicked it.
I'd look at the back of the leaves as their appears to be a long brownish line from the tip to the bottom of the leaf, looks like some insect or leaf sucker has buried it's eggs or eaten leaf, laid eggs or whatever, use an eye glass if needed as some bugs are very small. like spider mites are the size of a pin head.
Once you find the culprit, either squash by your finger / thumb, wipe both sides of each leaf with a cotton swab dipped into a week solution of methylated spirits and water. IF all else fails, remove the damaged leaf and keep an eye on the other leaves or take the leaf to the garden store and ask the plant attendant for something to hep with the problem you have found. Plants are tougher than we think and it should soon recover
Hope you can soon find the name of the bug and the plant can pick up and grow again as before.