Howdy Eco,
It could be that your first cell has a different impedance from the rest.
If all cells are the same, you'd see a DC voltage across each one that is fairly similar.
You could liken this to the charging of batteries, as each of their cells should be closely matched during charging.
If the affected cell is running cooler than the others, or has a lower terminal voltage, that might be as a result of a richer concentration of chemical additives in that particular cell.
I have yet to try a multicell series arrangement, but I can imagine that I will see similar problems that would need to be ironed out.
Best wishes,
Steve-tee.