Currently, the API to update specifications https://developers.vtex.com/docs/api-reference/catalog-api#put-/api/catalog/pvt/specification/-specificationId-, in its documented data, presents the field DefaultValue as a possible property that can be updated.

However, changing this field after the specification has been created yields no results.

Simulation

1 - Fetch data, using the GET verb on the same route, for a specification that has a DefaultValue registered in it https://developers.vtex.com/docs/api-reference/catalog-api#get-/api/catalog/pvt/specification/-specificationId- OR create a specification with one.

2 - Using the response body as a payload, try changing the DefaultValue field

3 - The response body will show no changes, despite the different informed request body.

Workaround

Manually change these values via admin UI.