You say "fresh rebuild".
Does this mean a new bearing right next to the leaky seal?
If so, did you remove the side caps/seals on the new bearing?

I think you will find that gearbox oil gets delivered to the LHS of the bearing and if the side caps/seals of that bearing were not removed when installed, it causes the sprocket seal to leak.
You will need to pull out the cassette gearbox, (leaving the motor in the frame) and pull that bearing out I'm afraid.

Or it may just be a leaky seal? Lets hope so.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...