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It's possible that it could be something else, I'm not an expert on plant problems.
The two things I am sure it is not are over watering or lack of nutrients. By flushing your root ball and resetting your PH to 6.5 you give the plant a chance to heal itself, with no downside.
Then start over by feeding the root ball, not the above ground part of the plant.
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{it's not heat stress either, definitely a nute problem of some kind. As kimber said, it's probably the OF soil. Good stuff but sometimes too much nutes for a young plant.}