Siblings of $ref are incorrectly dropped in resolved OAS 3.1 specs

[triblondon]

Describe the bug

When validating an OAS 3.1 spec using a custom function, my custom function receives the resolved schema without siblings of the $ref. The following should be semantically equivalent in OAS 3.1 but they are not:

This works correctly, offering me an object with all the properties of the referenced BaseOrganization, merged with the local description and unevaluatedProperties properties:

components:
  schemas:
    BaseOrganization:
      type: object
      properties:
        name:
          type: string
    OrganizationForCreateOrUpdate:
      allOf:
        - $ref: "#/BaseOrganization"
      description: An organization entity object for create or update operations.
      unevaluatedProperties: false

But, in contrast, the following doesn't work, and my custom function receives just BaseOrganization. The local properties are ignored:

components:
  schemas:
    BaseOrganization: # as above
    OrganizationForCreateOrUpdate:
      $ref: "#/BaseOrganization"
      description: An organization entity object for create or update operations.
      unevaluatedProperties: false

In OAS 3.1, $refs are supposed to be able to have sibling properties, which are merged with the $ref'ed object.

To Reproduce

Write the second of the test schemas above into an OAS spec, and test it with the following rule

given: $.components.schemas.OrganizationForCreateOrUpdate
then:
  function: truthy
  field: description

Expected behavior

The description property is expected to be defined and truthy but it's absent.

Environment (remove any that are not applicable):

• Library version: @stoplight/spectral-core@1.21.0