Windows 7 64bit
Blender 2.80 #552b2287db86
When using any of the pose Tween operators (Shift/Ctrl/Alt + E) / (Pose > In-Betweens)
The operator only considers the final absolute keyframe, and not what type of action it is or the layered result.
If you open this file and try to run any of the in-between operators, you will see the bone jump far from where it is, to the center, based on it's curves' exact keyframe values.
(Also, would it be difficult to also add object In-Betweening? Unlike bones, if you animate with objects, your only option ever is/was to manually insert keys in the graph editor)