Hi Aeterna Games,
First I'd like to confirm that your separation and offset are set correctly after using tile set champion. When you upload a tile set into PB, and that tile set has been modified by tile set champion, make sure that the separation is double what you put for the buffer size, and that the x and y offset are equal to the buffer size.
So for example, if you uploaded your tiles to tile set champion and made a the buffer size 2, then in Platform Builder, the separation should be 4 and the x/y offsets should be 2.
If these are correct and you are still having problems, let me know.