I pushed update 6.77.0 that should fix this issue!
Right now only 1 series is supported for the 2nd y-axis. Instead of adding another series to the 2nd Y-axis, can you not add those series to the 1st y-axis and add the series that is on the 1st y-axis to the 2nd y-axis? (so you end up with 1 series on the 2nd y-axis)