The runtime doesn't actually matter. The behavior is the same across all runtimes.
If you have any mixing in the animation queue, and you have animation1, which has a bone with a -1 scale, and then animation2 with that bone with a +1 scale, the mixing will cause it to smoothly interpolate the scale from -1 to 1. That's just how AnimationState mixes animations.
Unfortunately, it means scale -1 will pass through (theoretically) -0.5 and 0 and 0.5 before going to 1.
If you want your sprites to look solid, you don't want them to flip like a piece of paper every time you switch animations.
There are ways to tell the Spine system how to interpolate between 2 keys (linear, curved, stepped), but no way to selectively tell AnimationState how to mix certain dopesheet items/Timeline objects between animations.
This problem was actually present in the hero sample skeleton but I didn't get it until now.