Hi,
if I do
skeleton.ScaleY = skeleton.ScaleY + 0.001
the skeleton FLIP up/down
ScaleX work right.

If I set ScaleY = xxx
it work right

I think because XNA/Monogame has Y inverted in Spine2D (I don't know the why)
If I set ScaleY=1 then I check the value it is converted in negative: -1

If I scale adding value to the rootBone it work right with x and y

best
Cristian