Maybe your mechanic has replaced the seal before, and it looked good with a ground run, but after the next flight, it leaked again…
The gearbox pressures in the Rolls Royce engine are greater during flight that on the ground. The air/oil separator gear is N1 driven, and the gearbox pressures are higher at flight N1 speeds.
Or, like the others have said, the mechanic may like to fly.
What’s wrong with that?
Happy Christmas. May all your mechanic’s have this work ethic.