There is a message in
/var/log/messages on OBAS node like
Jul 4 11:25:14 pdns: Removed from notification list: 'domain.tld' to 18.104.22.168 (was acknowledged)
Where 22.214.171.124 is the slave DNS server IP address.
Notification list is an internal PowerDNS entity. Notification list contains a list of domains that needs to be synchronized. After a succesful synchronization a domain is removed from the list. It is absolutely normal for a messages log to have such messages logged. Removal from notification list does not affect nor availability of a domain neither its DNS zone, it just means that the DNS zone is up to date.
It is safe to ignore such messages.