From the The Building Coder blog:
Moving Items Inside a Detail Group
August 17, 2017
By Jeremy Tammik
Fair59 solved another issue in the Revit API discussion forum thread on moving detail group only moves its location point, not detail items inside:
Question: I’m trying to move Detail Group element to a point that was picked, but only the Location of the group moves, not the detail items inside.
Answer: To move the entire group, you can use:
XYZ moveVector = new XYZ( 5, 0, 0 );