I'm also getting the same behavior: The RoboDK ribbon is blank/empty in an Inventor assembly file, but is fine in a part file.
Inventor: 2021.2.1, build 289.
RoboDK dll info: 1.6.7461.19605
Inventor: 2021.2.1, build 289.
RoboDK dll info: 1.6.7461.19605