An object in NetXMS always has to have at least one interface, where its Primary IP will reside.
If an object has all information sources disabled (nxagent and snmp), the object is created with a synthetic interface to hold its IP.
Next up, when an interface is created, NetXMS looks to place it in a subnet.
NetXMS will look for a matching subnet to place the IP in.
If a subnet doesn't yet exist, a new one will be created - specifically a /24 in this case, since the synthetic interface has a default /24 mask.
You can influence this by changing the server configuration parameter DefaultSubnetMaskIPv4 (or DefaultSubnetMaskIPv6).