Unfortunately, this is a very well-known and very well-documented bug that has existed since 9.1 dropped last June. This and other store-bought mogs as well as RAF rewards will appear and disappear without rhyme or reason. There is not a fix as yet, but they are working on it, as evidenced by one of the CS SFAs announcing that they have finally replicated the instance, which is a huge step in the right direction.
Until that time when a fix has been announced, you are using the mog at your own risk.