There is NOTHING to stop the oil from draining from the head area to the sump, it needs to, there is a big hole at the rear where the oil goes back and there is no restriction/valve.
The only "anti drain" so to speak on the head is in each tappet, so in theory they hold the pressure so keeping the contact so they don't rattle, in practice most take a few seconds to pump up each day, a lot more so when old.
tabetha