Sankey Diagram only displays the first trace to a node

Relevant codepen: https://codepen.io/DominicWuest/pen/KKJBvzm

You can see how, when hovering over the links, the full traces are displayed.
However, when hovering over the rightmost node (C1), only the first trace to A1 is displayed even though the traces from A2-A4 should also be highlighted.

Is this an error on my part? How do I fix this?