There are two processes that run on the Secondary:
1. hardening of the log
2. REDO (replaying of the log)
Not sure if this is what you're running into, but the Syncronization state you see in the dashboard and through the DMVs only applies to the hardening of the log. If for some reason REDO has latency, the data on the Secondary will not be up to date, even though the sync has committed and reported back to the primary that all is well.
REDO latency issues such as this can cause problems when backing up on the Secondary, because the backup will include only the point up to which the log has had the REDO applied. REDO latency may also be the cause of the issue that you're seeing.