The pose library blending code assumed that quaternions would be keyed
in order of `array_index`. This is normally the case when creating a
pose asset, but when manually editing a pose asset Action this
assumption may not hold.
The code still assumes that all FCurves of a single quaternion are
sequential, i.e. are not inter-mixed with other keyed properties. They
just no longer have to be ordered by array index.
Ref: blender-addons/issues#104591
5532d05622