Copy paste is not working inside of UMG Expandable Area widgets. You can Copy but the Paste option is grayed out. I don’t see why this shouldn’t work when moving things into the header or body works fine.
I was able to reproduce this issue on our end. I have written up a report and I have submitted to the developers for further consideration. I have provided a link to the public tracker. Please feel free to use the link provided for future updates.
Link: Unreal Engine Issues and Bug Tracker (UE-48454)
Make it a great day