Thanks, received your package and could reproduce the problem, as described via email.
I filed an issue ticket here:
https://github.com/EsotericSoftware/spine-runtimes/issues/1321
The problem with remaining animation leftover rotations when transitioning to an empty "Write Defaults" state is now resolved.
You can download the latest unitypackage here as usual:
Spine Unity Download
Unfortunately I see one remaining problem of the transition to the 'Default' state being a hard cut, because Unity's animation controller returns clip.weight=1 all the time and then instantly clip.weight=0 instead of e.g. a proper clip.weight=0.9, 0.7, 0.5, etc during the transition.
However, I could not yet find a way via the Unity API to query the exact used transition weight. I will let you know if I find out anything regarding this problem.
Nevertheless, I hope that the situation is now at a state which allows you to work with, or work-around: you could define a simple animation in Spine which has only the setup pose rotations set, and assign it to the Default state in the Mecanim controller. This should then lead to the desired result.