I did a fresh installation of SQL 2016 SP2 with 2 Cluster Nodes and 12 databases in the AG. It all worked fine without any "UpdateHadronTruncationLsn" message until I did...
I get the message "UpdateHadronTruncationLsn(8) (force=0): Primary: 0:0:0 Secondary: 37:336:1 Partial Quorum : 0:0:0 Aggregation: 37:336:1 Persistent: 37:336:1" exactly each 30 minutes.
I do have some other AON installations which work...