Its seems to be a bug in the rendering engine.
I made a watercoastline with the water cornerparts. In CentrED everything shows correct. But IG the small cornerparts were not drawn. In punts worldeditor they also disappeared, so i could correct the mapheight of the cost until they are drawn.
Ground z=0 "x"
waterunderground z=-15 "o"
xxxxxxxxxxxxxxxx
xxxxxxxxOooooooo
oooooooooooooooo
And then staticwatertiles with z=-5 above the underground. The watertile at place marked with "O" disappears IG but not in CentrED.
Thanks for that example to reproduce it. I'll take a look. The rendering is the most complicated part so it could take some time until I'm able to fix this.