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Argh, that sucks! My best guess is that it could be: Cylindrocladium leaf spot disease. There are multiple types of this disease.
Likely a virus
I had a similar issue with my sugar apple seedlings when they first popped up. I found it was a very tiny black worm/caterpillar chewing lines in the leaves. Your problem looks nearly the same and I would be it is a insect eating the leaf.
Yes, it is a leaf miner.Try an Azadirachtin-cpntaining product, on the leaves and drenched into the soil. If there is still a live larva there, the Azadirachtin will prevent the larva from molting.Won't fix any damage already done.
Fungicide would definitely work and letting soild dry out is also key to keep it from spreading.