LÖVE

I drew a diagonal black line on each of the tiles in the source image and got this result: This means that the bottom and right tiles are not stretched. In fact they are the only tiles that are shown correctly. All the other tiles are overlapping. That means somewhere in the spritebatch you position the quads incorrectly (factor of 2 somewhere)

Well I know exactly where that comes from. I'll leave it as an exercise in order to save myself from pointing out the ridiculous mistake.

Thanks for the quick response.