Additional Property Slug Source works only when the following two conditions are met.
Use a different source. Additional Property Slug Source must be set to a different source than Property Slug Source. If both settings use the same source, the mapping does not apply.
Confirm your property page URL base. Most sites serve single properties at /property/<id>/, but the base can differ by site. If the base is wrong, the property page returns a 404 no matter how the slug is configured.