This happens when a the scale x/y is separated on a mirrored bone (e.g. at -1.0 width) and the animation is used additively.
The bone will reflect and any scale changes invert (shrinks when grows, grows when shrinks). Reflection will also be caused if there's no change in scale and only x/y is separated.
This behavior did not happen on 4.1.24 stable. I'm pretty certain it's unintentional.
Here's some tests I ran on the chibi stickers pro example.
First, control. I did not separate the channels. This works as one would expect it to behave,
I increased both bone's width to 1.2 (-1.2 and 1.2). On playback and used as additive it behaves correctly.
Here's the same, but after separating x/y scale
And here's without scale change (left as -1 and 1). Showing that the left arm is reflected nonetheless
Non-additive behavior is unaffected, this exclusively happens as additive and when scale x/y is separated. It only affects mirrored bones.
I am not aware if this extends to other transforms such as shear, or if there's any glitchy behavior with separation in general.
Thanks for the continuous support and work you put into Spine.
