Hi together,
if we duplicate a group of threaded text frame items to a different Illustrator document the story's threaded text frame items will not be threaded anymore in the target document.

If you duplicate the group in the same document the threaded text frame items remain threaded.

All details here at Adobe Illustrator Scripting Forum:

[BUG] copying threaded text-frames.
Silly-V Mar 22, 2019 9:30 AM
https://forums.adobe.com/message/10991060#10991060

One has to rethread the text frame items in the target document.
That should not be necessary.

No issue if we copy and paste grouped and threaded text frame items using the menu commands.

Regards,
Uwe Laubender
( ACP )