I had the same problem installing a new bathroom sink a few years ago... until it dawned on me that no matter how tight i cranked the nut down that rubber gasket was not completely sealing the threads underneath it on the tailpiece. Disassembled it ,filled in the portion of the threads that the gasket covered with plumbers putty, reassembled, and it hasn't leaked since.
they is cheap for sure i went to metal no leaks
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