Thank you BarbiJKM and txny. I figured out my issue based on BarbiJKM post. It looks like Z class is not available with the multiple destinations. If I book two separate one way tickets, it shows the Z class availability. The reason we're flying from MOZ is we're spending a few days there.

txny, thanks for sharing your experience. It sounds like it's worth taking the chance especially with cost per kilo that BarbiJKM quoted.

Did you notice any difference in the seats for Y for Z class?