Overriding (disabling) a tab using new YAML syntax doesn't work.

Steps to reproduce:

1. Create a dialog like

   form:
     tabs:
       tabMain:
         label: Main
         fields:
           - name: title
             class: info.magnolia.ui.form.field.definition.TextFieldDefinition
             label: Title
       tabCookie:
         label: Cookie
         fields:
           - name: showCookieMessage
             class: info.magnolia.ui.form.field.definition.CheckboxFieldDefinition
             defaultValue: false
             type: Boolean
             label:  Cookies
             buttonLabel: Set Cookies

1. At other dialog, include the dialog above and override the tabCookie:

   !include:/my-module/dialogs/components/myDialog.yaml
   form:
     tabs:
       tabCookie: !override

1. Check that if you open the dialog, tho the tabCookie shouldn't appear, it does.

Actual result:
The tabCookie is visible

Expected result:
The tabCookie is not shown

Dev notes:
Using the old YAML syntax, the tab is hidden:

form:
  tabs:
    - name: cookie
      label: My Cookie tab
      fields:
        - name: showCookieMessage
          class: info.magnolia.ui.form.field.definition.CheckboxFieldDefinition
          defaultValue: false
          i18n: false
          type: Boolean
          label: Cookies
          buttonLabel: Label button