I downloaded your example and, I don't believe a 9-patch is the right object to use for it. See the reference link about what a 9 patch is:
Using a 9 patch, any time you scale the platform, positions 2, 5 and 8 in the above diagram would be stretched or tiled (if you set the property to tile) any time you scale it horizontally and positions 4, 5 and 6 would be stretched or tiled if you scale it vertically. Positions 1, 3, 7 and 9 would never stretch causing the strange results you are seeing.
If anything, I would suggest you either just use a simple sprite object, or create a platform image which can be easily tiled. If you create 9 different images (4 corners and 5 easily tile-able center images, you could then combine them and use the 9 patch object set to tile. You would just want to make sure the object always scales to a multiple of the height/width of the tiles.