Hi Vincent,
Both you and VMware are correct in regards to this situation, firstly your storage is not supported for the ESXi 5.5, But you were right with the thought the incorrect "used storage" does seem to be a Bug , I was able to reproduce the problem with a different
storage model and the same issue. I am working with VMware to see if a problem report can be filled. I will provide you with more details, If it is confirmed to be Bug by VMware Engineering.
Thanks,
Avinash