I have a strange problem with setting keys in the Motion Layer Editor ...

I made a new project, copied the Party Girl into the scene from the content manager. Then I opened the Motion Layer Editor to add a key by rotating the arm like this:



This creates an animation key in the timeline ... but there are no inbetweens created  ...  When I playback the animation it "switches" between the pose in frame 1 and the newly created key....
It behaves like a "poseswitch" .... as if "linear interpolation" is turned off or something....





Maybe I accidently clicked on a button or activated a checkbox somewhere and set IC into some kind of a "switch mode"?? IDK ...
I tried the same in IC7 (Trial) and there it works as you'd expected...

I'm a little bit confused ....
Please Help! :)