As per TIS (and maybe whoever worked from the dealership on your car maybe didn't observe the procedure), the nut has to be unscrewed gently, and the connection has to be examined for damage & dirt. When reassembling, it has to be thoroughly cleaned. The O-ring has to be replaced if damaged. Tightening torque is 4 Nm.

So, if it was over torqued, not cleaned, or the O-ring was bad, there is no seal.
4 Nm is ridiculously low. I have a small torque wrench, and when I tightened the same connection at the rear, at first I thought the wrench isn't good, it's that low.......

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.