I've had this happen before
Usually happens if I haven't used the Leva for an extended period, in the order of weeks/months. I also have an NS Oscar which is my daily driver. I typically only use the Leva on weekends at the most
If i haven't used the Leva for months, it usually will leak through the exit hole as you described. Eventually after a couple of cycles and flushing of water through the grouphead, the water coming up lessens till eventually it will no longer occur.
My SWAG: (sudden wild a.. guess) is that the materials eventually come up to temperature and the seals and metals swell up. As an aviation buff, this is somewhat reminiscent of how the SR-71 blackbird behaves on initial takeoff (an internet search should provide details on this); fuel leaks so much that a tanker is reqd to top off before it goes on its merry way.
Maybe other folks here can confirm with a more scientific / physics explanation but since my seals are relatively new and it eventually stops, I'm not too worried about it. I consider this a "feature" of the leva