Can you try to create a tempate with E100o adapter instead of vmxnet3 and try the customization
That will narrow down if the issue is with vmtools/vmxnet with 2016 customization o something else.
can you try using sysprep from the guest instead of using the customization wizard from vSphere.
Get the template server ready with tools and updated HW version etc.. run sysprep .. generalize and shutdown.. convert the vm to template and deploy from the template.
Although it will not fix the original issue, it does the same job unless you have some additional mentioned in vCenter customization specs.
we might need trivia or vebose vcenter logs for troubleshooting the issue if we choose to..
So I tried to start over with the template with another iso installation.
Every step was done manually and then converted to template.
Now customization works with windows 2016.
Thanks for the replies.