When booleans are blended together, they are blended as floats, and any non-zero value is treated as true.
While this is not ideal, and there are good arguments for treating 0.5 as the point where the blending goes from true to false, this behavior has been the same for at least the past 12 years, and there are countless released games that rely or may rely on the existing behavior. Therefore we will not be changing this behavior at the moment, as we deem the risk to break existing games is too high.
Here are some workarounds you can use to achieve a similar result:
* Using actual flipped sprites
* Animating a float on a MonoBehaviour and using the OnDidApplyAnimationProperties callback on the MonoBehaviour to convert this float to a boolean, then applying it to FlipX.