Presumably ifupdown configuration happens sometime after.
The above write_loopback procedure is triggered again after install from
55netcfg-copy-config, if "netcfg/target_network_config" is set to
"nm_config" or "loopback". (Wiping then writing just a loopback entry).
Maybe you need to preseed this setting to "ifupdown" for it to work as
expected?