In multiple DNS server environments, not all IP addresses get their hostname resolved by IPAM.
All versions of IPAM
You may configure default forwarders such that it forwards the recursive DNS queries to all other DNS servers, other than the default/internal DNS server.
For more information about forwarders, see the Microsoft website.
The hostnames of IP addresses are resolved only if the default/internal DNS servers host the corresponding zones. The DNS query does not find the other DNS servers present in your network. Because of this, IPAM could not resolve the hostname for certain IP addresses.