Behaviour of charts beyond polar circle. Both principles - dynamic vs. static - have their merits so there's no golden rule here. I personally choose MC to be continuous all over the globe, to avoid discontinuous 180 degree jump when Asc changes direction (dynamic over static principle), so beyond polar circle sometimes it may be on ecliptic the point that is below horizon. Another point for this zenit rule is that sticking to 'ecliptic longitude of MC' to be the point above horizon, this static principle quite often incorrectly places the planets that are in lower culmination into 9/10 house, while beeing in lower culmination they should be in 3/4 house.
...the same two charts, but here presented as equatorial rectangular map.
white line in middle - equator
yellow line - ecliptic
green line - horizon
red line - east point/zenith circle
blue line: asc/zenit circle