I think it's fair to assume that cocos2d-x's tilemap renderer is a direct port of the one in cocos2d-iphone. If true, they both have the same restriction of a maximum of 65,536 vertices (16,384 tiles) that can be displayed (not counting empty tiles).
Your tilemap is 400x400 = 160,000 tiles assuming there is only one layer and there aren't any "empty" tiles (empty == tile locations with GID value 0). That means about ten times the number of tiles cocos2d will/can render.
Cocos2d will just render up to 16,384 tiles and then stop, the remaining tiles will not be rendered so you'll see the background color (default: black).
A common but awkward workaround is to split the map into several TMX files and align them in code.