I didn't miss it, I just didn't mention it specifically because that's not the way Scribus works.
An object's rotation affects both the object and its contents at the same time regardless of when you add the content in respect to when you rotate it.
It's consistent behaviour that is easy to learn. The rotation always applies to the whole thing throughout the object's life.
Having said that however, some kind of "Make the current orientation the zero degree rotation" option might be an addition but then I imagine that would have knock-on consequences.
For instance, if you create a rectangular frame, then rotate it, then add text or an image (which will be rotated), then "reset the orientation": Would the text or image be rotated back to horizontal or would it stay rotated? Different people might want different things and so you'd need two rotation attributes for each object; one for the shape and one for the contents.
And that could be more confusing than having a single rotation attribute. Would both be locked together so if you rotated the object again the contents would also rotate by that amount or would the contents be separate and stay where they were? Or would you have yet another option to lock/separate them, thus adding even more complexity?
The same would apply if the text or image is made horizontal when it is first added regardless of the objects prior rotation. You would, again, need two rotation attributes.
Or the orientation of an object could be "reset" back to zero degrees after each rotation but that would have its own conceptual problems, especially for beginners. ("Didn't I just rotate that?") And what if the rotation was - maybe accidentally - by a tiny amount that wasn't very visible initially? Rotation would show as zero but everything would look a bit "off". ("This software is broken.")
It's a tricky issue with no real common ground between the different ways of doing things.
It's a case of: One easily understood rotation for the whole thing VS. Two separate rotation attributes that can get out of line and need more user attention/control.
I can't say what the average user would find easier but my guess is the first one. (It wouldn't be the first time I was wrong though.)