In our environment setting a host to build either fails to add a DHCP entry to dhcpd or leaves incorrect ones behind if the subnet in question is bigger than a /24:

subnet 165.212.8.0 netmask 255.255.248.0 {
option routers 165.212.15.231;
option domain-name "perimeterusa.com";
option domain-name-servers 165.212.12.12;}

We then have a matching entry in foreman.

If we try to build a machine in 165.212.8 it creates the entry.
If we try to build a machine in 165.212.9 it fails to call the proxy to create the entry.

Sometimes we get other behavior like it creating it in .8, then in .10 and not removing the .8 entry in DHCP.