There is a cavity higher up in the ZA's case that holds oil to keep the main shaft and output shaft bearings and gears lubed. Initially the cavity gets filled up first when you fill the fluid thru the fill hole, at which point the cavity overflows and fills the trans area. Once it's running the cavity stay filled by a groove in the top of the case that catches fluid that is flung off of theclutch bell. The tip of the drain plug seals the bottom of that cavity and provides complete draining of the cavity when the plug is removed.
Old school ZA's had the o-ring plug in which the o-ring seals the bottom of the cavity and the plug threads in to threads in the bottom of the case.
The new style simply uses a bolt that threads in to threads that are where the o-ring rested in the old style, so the threads privide the sealing of the cavity, as well as holding the plug in, as there is no threads in the very bottom of the case.
Your fine. Nothing to worry about.