We get log backup from the secondary node. But now I see that log_since_last_log_backup_mb in sys.dm_db_log_stats returns null for databases which in Availability groups, so the log backup procedure(of Ola Hallengren) with 
@LogSizeSinceLastLogBackup = 25,@TimeSinceLastLogBackup = 21000 limitations
works on those databases. But the backup process is currently working on the secondary node. 
But I could not understand why it returns null? Have you encountered such a situation? Following pictures show result  of log size since last log backup in primary and secondary node
[![enter image description here][1]][1]

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/qWzdQ.jpg
  [2]: https://i.sstatic.net/hC5up.jpg