I have a template with server 2019.
I have created some servers from template and used my 2019 customization file.
But servers have not been customized.
In customization file I have not inserted any license key, because I still waiting to get one.
The template is also not licensed.
Could that be the reason?
My 2016 templates/customizaion is working fine.
You can check the customize-guest.log file in the VM. And perhaps give some more details about the environment (versions etc.)
Vcenter 6.5
File I will look into when I'm back at work
Maybe not completely relevant but it contains valuable information: VMware Knowledge Base