After Upgrading to ESX Version 6.5 and 6, serial number of the ESX host is updated with Chassis serial number.
-Servicenow discovery looks at the value serviceTag on the Vcenter to populate the serial number of the ESX server. This is a VMware bug where they have mapped the ServiceTag value with chassis serial number in versions starting from 6.5 and 6.7.
Vmware confirmed this is a known issue with VMWare ESX 6.5 and 6.7.
VMWare has resolved the issue in ESX 6.7 U1 and has a pending fix for version (ESX 6.5) to be released in November’s 6.5 Patch 03