To get on top of some of the "peculiarities" of Ducati, We have to be inventive

But I guess that, even with the o-ring at that place (without interfering the shimming) oil can get through the splines of the shaft and when dismantling the sprocket the source isn't Always clear.
In my opinion, build-up of crankcase pressure is the primary reason to push oil higher than the standard level.
In some Ducati engines that pressure was used to lubricate, but not in the bevels

Good of You to keep a record on the work You've performed on the bike.
ciao
ducadini