Availability Group Transaction log issue

We're having an issue with an AG where the the log backup does not appear to truncate the log. symptoms

• Run full backup
• Run transaction log backup
• DBCC Loginfo shows all VLFs with a status of 2
• sys.databases.log_reuse_wait_desc says LOG_BACKUP
• OPENTRANS indicates no open trans

All backups are being run on the primary.  Any ideas?

thanks