I have had a couple of instances of the following with the Legend native windows app:
I’ve had a Board containing two panes. In one Pane’s document list, I could not collapse the second level of children in a document’s display. In the second pane, it worked fine. The root leve, and third level could be expanded/collapsed, but not the second.
I tried it in the Web App Legend, and the same board’s document list showed the same characteristics.
Cloning that board yielded a new board with the same issue.
Deleting the board and recreating it with the same name seemed to work fine afterwards, so it seemed to be linked to a specific board.