When creating a NAD policy and choosing "localnet" option, if you than switch to the YAML view you get a YAML without the additional spec.config fields needed to config a localnet network.

v4.15.0

100%

1. In the UI go to networking -> Network Attachment Definitions.
2. Click on the button to create a new NAD.
3. Under "Network Type" choose "OVN Kubrenetes secondary localnet network".
4. Click "edit YAML" option on the top.

Switching to YAML view, the additional localnet fields are missing.

Having the localnet fields included under spec.config.

Same issue is valid for the "layer2" topology.