Resource based hostname not available when launching instance

I want to use resource-based naming for a new EC2 instance. When launching the instance, "Hostname type" is set to "IP name" and cannot be changed.

From https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-naming.html

The EC2 instance guest OS hostname type depends on the subnet settings:

• If the instance is launched into an IPv4-only subnet, you can select either IP name or resource name.
• If the instance is launched into a dual-stack (IPv4+IPv6) subnet, you can select either IP name or resource name.
• If the instance is launched into an IPv6-only subnet, resource name is used automatically.

My VPC is dual-stack:

Can anyone explain why this is?