My Research didn't find any answers to my problem:
Dragging and Dropping Subclips to the Sequence works as expected: Only the (shortened) length of the Subclip is dragged on the Timeline.
By Using "Automate to Sequence" the length of the original Clip (much longer) is dropped on the Timeline.
Is this by Design? I expected the same behaviour like dragging and dropping. With this behaviour its impossible to create quick rough cuts when you have Subclips in the project.
Workaround for me: The length of the SubClip is not ignored if you check the "Restrict Trims To Subclip Boundaries" at the Subclip Properties at the time you use the "Automate to Sequence" Function.
If you had to trim/extend the SubClip Boundaries afterwards, you have to uncheck the option for every subclip you used. So this really just i a workaround in my opinion.
So is this an issue for a bug report?