Would anyone be able to point me at why types aren't showing as non-nullable in generated clients?

i.e. I have the following:

        builder.Services.AddSwaggerGen(opts =>
        {
            opts.SupportNonNullableReferenceTypes();
            ...

But when I generate the schema I can see nullability isn't respected.

The Features property on FeatureCollection for example is declared as non-nullable here

But then when generating an OpenApi schema it has nullable: true

"features": {
  "type": "array",
  "items": {
    "$ref": "#/components/schemas/Feature"
  },
  "nullable": true // <--
}