The zorder property changes the color of stacked line traces. The following code pens demonstrate the problem. The only change between the various charts is, that the zorder property has been added to the traces. This should not have any impact on the colouring, but it does.

Stacked line traces (working / without zorder)

Stacked line traces (broken / zorder=1 on first trace)

Stacked line traces (broken / zorder on all traces)