Hey Harald (when you get back from vacation :grinteeth: ) and everyone!
I created a bug report for this here: https://github.com/EsotericSoftware/spine-runtimes/issues/1953
To repro:
1) New project with most current spine-unity runtime
2) Create SkeletonAnimation using Spine Examples > Spine Skeletons > Dragon
3) Press the "Initial Flip X" button on the SkeletonAnimation -> The skeleton doesn't flip (in Editor mode)
4) Run game, the skeleton is correctly flipped.
Workaround: In Editor mode, if you press "Reload" on the skeleton after changing the Flip, it then updates it to show the correct flip.
Skeletons that were previously flipped in my actual project are still correctly flipped in Editor mode. It just isn't updating them in Editor mode when I make a new change to the Flip value.
Thanks for the help as always, and hope you had a great vacation!