sequence.parent.yml

# Working 
field.instance.*.*.*:
  type: mapping
  label: 'Field instance settings'
  mapping:
    default_value:
      type: field_instance.[%parent.widget.type].default


field_instance.text_textarea_with_summary.default:
  type: sequence
  label: 'Defaule value'
  sequence:
    - type: mapping
      label: 'Default'
      mapping:
        summary:
          type: string
          label: 'Summary'
        value:
          type: text
          label: 'Body'
        format:
          type: string
          label: 'Text format'

#  Not Working schema
field.instance.*.*.*:
  type: mapping
  label: 'Field instance settings'
  mapping:
    default_value:
      type: sequence
      label: 'Default values'
      sequence:
        - type: field_instance.[%parent.%parent.widget.type].default
          label: 'Default value'

field_instance.text_textarea_with_summary.default:
  type: mapping
  label: 'Default value'
  mapping:
    summary:
      type: string
      label: 'Summary'
    value:
      type: text
      label: 'Body'
    format:
      type: string
      label: 'Text format'