I have a custom CRS centering on the Pacific ocean with the Mollweide projection:
+proj=moll +lon_0=-158 +datum=WGS84 +units=m +no_defs +lon_wrap=-158
My countries are split at the 22nd meridian, and look fine with a rectangular projection such as
+proj=cea. Something about the curves at the edges is messing it up.
Also strange is the zooming behavior - as I zoom in on the edges, the curves of Africa & Antarctica at the edges are filled in. It only looks this way when I'm fully zoomed out to see the whole layer.