Talked to another dealer : he confirmed that there may be effectively this leak problem since they've received in the box a R3 with the bottom of the box* <<
completely full of oil>> !
...
...However the dealer told me that there is nothing to worry about since it was only a problem due to a wrong tightening torque of a bolt/screw; he added that's part of the inconveniences specific to new models.
So from that day they'll make sure to double check this element before delivering to the customer and they expect that it should not occur again with the new deliveries given that Triumph has been obviously informed.
* this kind of box as we can see here :