I have figured out what causes it and how to mitigate the problem, at least in my case.
I have a sprite which has 6 frames, each one a different solid colour. They happen to be 256x 256
The first frame is blue, second green, third red etc.
I place a sprite and resize it for my needs and set the frame to 0 (blue). When i preview its fine but on export i see the edges of the sprite go from blue to green. Changing the frame to green the edges fade to red and so on.
The way i have found to stop this is basically to double the number of frames. So 2 x blue, 2 x green, 2 x red etc. Then by using frames 0,2,4 it stops the colour bleed.
Hope this makes sense and is of some help.