I am not sure if this was solved already, but I ran this today.
I exported a Spine project to json and noticed that my animation was saved with stepped interpolation, although in Spine itself it indicates bezier interpolation. See attached project for example.
One other strange thing I noticed is that when you select the scale keys in the animation the UI does not show any value for the scales in the key. I don't know the exact steps on how I arrived at this state.
Roland