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Hairline crack is possible, and I can even convince myself from your picture. I think you need to drain the oil and see the mating surface for the plug. If it is cracked, you will see the full thickness crack on edge. You also need to feel how that plug moves in the threads, as someone may have cross-threaded the plug and now you will never get proper torque.
Since new plug didn't help, it's either a crack or a mating problem.
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