Sankey diagram rearranging node positions

Hi everyone,

I was implementing a sankey diagram, and it seemed that some of the nodes were rearranged automatically regardless of what positions I assigned them to. As the figure below shows, nodes with the same colors should be aligned horizontally by their centers, but the first node is shifted to the vertical center, even though it was assigned to be at the bottom.

Any comments on how this happens, and any suggestions on how to work around this issue is really appreciated.

Thank you in advance!