In the preseed_default_finish.erb template no ipv4 subnet implies dhcp:

<% subnet = @host.subnet -%>
<% if @static -%>
  <%- dhcp = false -%>
<% elsif subnet.nil? -%>
  <%- dhcp = true -%>

The preseed_netplan_setup.erb template does not:

:dhcp => bond.subnet.nil? ? false : bond.subnet.dhcp_boot_mode?

This means a host setup with no subnet works everywhere else but creates an invalid netplan.

IMHO no subnet should imply dhcp.