When checking DNS settings for example.com, the service like https://mxtoolbox.com for primary and secondary servers reports the similar:
CONFIG_TEXT: Serial numbers do not match
- Different primary and secondary DNS servers are used.
DNS records on primary and secondary servers were created separately from each other, using two or more master DNS servers.
Note: Multi-master DNS scheme is yet to be implemented in Plesk. Please vote for it here
Configure slave DNS server(s) with Slave DNS Manager extension following the article How to integrate Plesk with a secondary (slave) DNS server?