How to query a list on AppSync GraphQL by a nested object

I am using AWS AppSync GraphQL and am trying to filter a list by a nested object's value.

My schema looks like this:

type Post @model {
    id: ID
    title: String
    content: String
    hidden: Boolean
}

type PinnedPost @model
{
  id: ID!
  userID: ID @index(name: "byUser", sortKeyFields: ["postID"])
  user: User @hasOne (fields: ["userID"])
  postID: ID
  post: Post @hasOne (fields: ["postID"])
}

I would like to run a query to list the PinnedPost for a user, but filter out the hidden ones, like so:

const pinnedData = await API.graphql(graphqlOperation(             
  listPinnedPosts, {
     filter: {
      userID: {
        eq: userInfo.attributes.sub
      },
      post: {
        hidden: {
         eq: false
      },
     }
    }
  }
))

I have updated the filterinput in my Schema through the AppSync Console to:

input ModelPinnedPostFilterInput {
    id: ModelIDInput
    userID: ModelIDInput
    postID: ModelIDInput
    post: ModelPostFilterInput
    and: [ModelPinnedPostFilterInput]
    or: [ModelPinnedPostFilterInput]
    not: ModelPinnedPostFilterInput
}

There are no errors associated with it, but the nested filter is not being applied as it will return both true and false values for hidden.

This question was sort of answered before:

Appsync & GraphQL: how to filter a list by nested value

but it is not clear to me where I am supposed to edit the mapping template to allow this. How can I achieve this result?