Is this correct?

BinaryCats

I realise flip x is changing, so this will unlikely result in a fix if it is incorrect

In this example there are 3 bones, Root, redbox (at 0,0) and blue box. The animation rotates the root 360

Here -1 scale is applied to redboxbone, it behaves normally (this is same as if there is no scale/flip applied)
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Here redboxbone has a flipx applied. You can see it rotates in the opposite direction. I guess I can understand why this is happening, as the axis are now flipped. Is this the same in other CG software?
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Nate

Bone flip always uses the world axes. I don't think any other software does this, because it isn't described by a transform matrix. It does everything as in your first GIF, then flips the position of the blue box over the world Y axis, using the red box world position as the origin for the flip.

It's weird. It's changing. Scale -1 will do the same thing as flip, as pretty much everyone expects it to. We've considered removing flip, since it just uses -1 scale under the covers. However, we may keep it because there's a difference in knowing a character is flipped versus it's scale is exactly -1. Usually this comes up when integrating with a physics engine. Maybe flip can be kept as a skeleton property, like it was long ago, which just flips the entire skeleton over the world origin. For anything else, people can use -1 scale.

Pharan

Additional consideration for the separation:
Skeleton flip may be used for persistent "facing state" of a character, to reflect whether the character's logic is making it face left or right.
A separate animated root bone flip may be used for transient, nonlogical flipping as in the cases of:

animating a turn animation (to include the character facing the opposite direction before facing the final one) or...
animating a spinning animation while allowing it to end facing the correct direction.

In case you're removing skeleton.flip, I guess the user can just have the root bone (for logic) and an intervening bone (for transient flips) before the rest of the hierarchy.