While it might be considered somewhat unconventional, StarUML does allow representing a method as a frame in an Activity Diagram.
In fact, this approach is supported by UML 2.x and is even recommended as a good practice to clarify the context of the behavior being modeled.
Since StarUML already supports using a frame to represent a method, it would make sense to also support displaying its full namespace (e.g., including the class name) within the frame for clarity and disambiguation.
Using annotations as a workaround is not ideal, especially because they don't update automatically if the class name changes.
Thank you again for your response!