Afaik this has nothing to do with PowerCLI, it's the way vCenter gets this information, and how and where some HW vendors make this information available.
With the CIM interface you can dump all the available information, and then find out where exactly the tag is hiding.
See what this returns
import-module CimCmdlets
$esxiHostname = "MyESXiServer"
$HostUsername = "root"
$CIOpt = New-CimSessionOption -SkipCACheck -SkipCNCheck -SkipRevocationCheck -Encoding Utf8 -UseSsl
$Session = New-CimSession -Authentication Basic -Credential $HostUsername -ComputerName $esxiHostname -port 443 -SessionOption $CIOpt
Get-CimInstance -CimSession $Session -ClassName CIM_Chassis
You will be prompted for the ESXi root password.
On some of our older IBM HW I get the following where the asset tag is expected.
But the information is there, just in another property.
As we already mentioned during our VMworld session, explore the CIM API.
There is a wealth of information in there