(what I thought when making this thread)
if a mesh has 0,0 scale, and you move the slot the axis arrows will snap back to the middle, this doesn't happen if its a image
(after a little bit of testing)
once I transform an image to 0,0 Scale, then make it a mesh (see above problem) then an image again, even after scaling back to 1,1 the image will be invisible, and untransformable (is that a word?). The image appears again if you reload spine
I'll make a note to check out what is going on, but it's pretty low priority.