default from $ref is not populated when the $ref is wrapped in a single-element allOf

[anya92]

Prerequisites

• I have searched the existing issues
• I understand that providing a SSCCE example is tremendously useful to the maintainers.
• I have read the documentation
• Ideally, I'm providing a sample JSFiddle, Codesandbox.io or preferably a shared playground link demonstrating the issue.

What theme are you using?

core

Version

6.x

Current Behavior

When a property is defined as allOf: [{ "$ref": "..." }] instead of a direct $ref,
the default declared in the referenced subschema is not populated into formData,
even with experimental_defaultFormStateBehavior.allOf: "populateDefaults" enabled.

On top of that, the select widget visually displays Cat - the first enum option -
while formData is {}. So the UI shows a selection the form data does not contain.

Expected Behavior

1. allOf: [{ $ref }] and a bare $ref are semantically equivalent for a single-element
   allOf, so both should produce formData = { "animal": "Dog" }.
2. Independently of (1): the widget should never display an enum option that is not
   present in formData. When the value is undefined it should render an empty /
   placeholder selection rather than silently falling back to the first enum entry.

Steps To Reproduce

Schema that works - formData becomes { "animal": "Dog" }:

{
  "type": "object",
  "properties": {
    "animal": { "title": "Animal", "$ref": "#/definitions/Animal" }
  },
  "required": ["animal"],
  "definitions": {
    "Animal": { "type": "string", "enum": ["Cat", "Dog", "Bird"], "default": "Dog" }
  }
}

Same schema with the $ref wrapped in allOf - formData stays {}:

{
  "type": "object",
  "properties": {
    "animal": { "title": "Animal", "allOf": [{ "$ref": "#/definitions/Animal" }] }
  },
  "required": ["animal"],
  "definitions": {
    "Animal": { "type": "string", "enum": ["Cat", "Dog", "Bird"], "default": "Dog" }
  }
}

1. Open the playground link below (start with empty formData).
2. Set experimental_defaultFormStateBehavior.allOf to populateDefaults.
3. Observe that the select shows Cat while formData remains {}, whereas the
   $ref variant shows Dog and yields { "animal": "Dog" }.

Playground links:

• working ($ref)
• broken (allOf)

Environment

- OS: macOS 26.5.2
- Node: 25.2.1
- npm: 11.6.2

Anything else?

No response

[anya92]

Also, I don't know if this is intended behavior, but this schema:

{
  "type": "object",
  "properties": {
    "animal": {
      "title": "Animal",
      "allOf": [
        {
          "type": "string",
          "enum": ["Cat", "Dog", "Bird"]
        }
      ]
    }
  }
}

does not work - when I select any value, the formData does not change and the selector remains empty. But once I change the "allOf defaults behaviour" to "populateDefaults", it works, even though there is technically no default value. So I think it should work without this option as well (playground).

[heath-freenome]

Looks like someone has already come up with a fix