I struggled with log truncation in SQL AlwaysOn backups. I researched with Veeam support and they pointed out this article that describes how the AlwaysOn nodes are evaluated for log backups.https://helpcenter.veeam.com/backup/vsp ... pport.html
In our case, the point #2 was affecting us in such a way, that log backup did not happen on the primary node, since there were files in the temp directory on the secondary node. Hence, logs were only backed up from the secondary node with Copy only and logs did not get truncated.
If this is affecting you, you should see for the active node this message (not a warning or error).
“….Skipping databases to be backed up from another SQL Server node…”