Each time an ESXi host is added to the vCenter Server, the vCenter Server's IP address is recorded on the ESXi host. If the host is behind a NAT and the vCenter Server is on a different network (and thus the address is not accessible due to the NAT), the host disconnects from the vCenter Server after about a minute.
For example, the vCenter Server system might be on the main network (10.10.10.0), and some ESXi hosts might be behind a firewall in the 192.168.x.x network. If the hosts cannot reach the main network, they will be disconnected from vCenter Server.
VMware vCenter Server
VMware vSphere ESXi
Using NAT between the vCenter Server and ESXi hosts is an unsupported configuration with no workaround.