After I completed zone 49 and I entered zone 50 by clicking "next zone", I exit the zone to the isle view, without completing the zone 50. I noticed that there is no zone 50 in my isle view and there is no flag on zone 49. And I was able to click zone 51 and conquer the whole isle from zone 51(As shown in the picture). Zone 50 is playable by finishing zone 49.
no mod installed.
I think I will just leave zone 50 to be finished later.
Maybe after a new patch coming out,