Your system has a one-way valve that allows outward flow only, and a oil separator that acts like a maze for any entrained oil droplets that try to leave the crankcase. This (normally) small amount of oil then drains back into the crankcase instead of entering the engine intake.
On Ducati street bikes the one-way reed valve is located at the engine case opening. During certain conditions of high engine rpm and light load, more oil is entrained so the reed valves’ small drain holes have difficulty returning the additional oil to the sump quick enough. The oil separator can fill with oil and get sucked into the intake tract. Higher compression engines or worn piston rings have more blow-by, so the likelihood of oil being puked into the airbox is greater.
So it's possible that the drain holes are partially plugged and can't keep up.
One way around this problem is by hollowing-out the reed valve to remove the constriction on drainage. Alternatively, STM valveless breather connector can be used.
Then use a one-way reed valve at the airbox connection. A 748RS remote breather valve works well here, see below.
Photo by DukeDesmo