If a schema extension is defined for a schema class that consists only of fields of a complex property, the property name without the name of the extension is used for a PUT request when serializing the json data. The name of the extension is then also missing in the "schemas" block.
This is a product defect (32696).
This will be fixed on a future release of the product. If you require this immediately corrected, please contact support for a hotfix referencing the defect ID 32696.