FWIW, Z class is actually the second cheapest business bucket on LX. It goes in the following order:
C > D > Z > J
In respect to your LX coded OS flight, I suspect Z class on LX probably maps into J on OS in this instance although be aware that another FT'er (Guy Betsy) recently reported some issues with an MS flight which was on OS metal in another thread.
I have a sneaking suspicion the mappings are not correctly set up so I strongly suggest you PM Joanna (or e-mail the usual address which also gets to her) and let her know your predicament including full details including PNR, ticket number etc. "Z" class does not exist on OS so logically it should map to "J" and I have a feeling it might be this mapping has failed and therefore may require some manual intervention to resolve.
Don't forget to update us with what happens.