If the power is lost in the middle of the write operation, only the last completed byte will be written. The incomplete byte will not be updated. The FRAM updates the memory after a byte of data has been clocked in. You can find this documented in the FRAM datasheets under write operation.
Yes, I have seen that in the datasheet. But I did not find any statements regarding any potential side-effects of power failure, such as reduced data retention.
For example on NAND, if power fail while writing a page, the page might end up as complete garbage or it could look correct but suffer from horrendously bad data retention (you might read back valid data the first time, but subsequent reads are corrupted beyond recovery).