That is a limitation of describing a matrix using SRT. When you scale the bone, image compensation tries to scale the image the opposite amount. This works for uniform scaling, where you scale both x and y the same. When you scale in only one direction, if the image is not aligned to the bone, the image's x and y scale properties cannot deform the image in the necessary way to keep it from being affected by the bone's scale. This is because the image is only described using SRT and by scaling the bone when the image is not aligned, you are trying to apply skew to the image. Skew cannot be described by SRT.
TLDR; this can't be done in Spine unless you align your images to the bone (eg the rotation of your image must be 0, 90, 180, or 270). Even then, 90 and 270 will compensate on the wrong axis. I could fix that but this is quite a rare task (you are the first to find it, congrats!). 🙂