Can't see nested elemets in API call

Post’s schema.json looks fine.

{
  "kind": "collectionType",
  "collectionName": "posts",
  "info": {
    "singularName": "post",
    "pluralName": "posts",
    "displayName": "Post",
    "description": ""
  },
  "options": {
    "draftAndPublish": true
  },
  "pluginOptions": {},
  "attributes": {
    "title": {
      "type": "string",
      "required": true
    },
    "body": {
      "type": "text",
      "required": true
    },
    "tags": {
      "type": "relation",
      "relation": "oneToMany",
      "target": "api::tag.tag"
    }
  }
}

but I don’t see posts as relation against tag, when accessing http://localhost:1337/api/tags?populate[0]=posts

In this case, posts and tags supposed to be in many-to-many relations.
As Post has all 3 tags, in the screenshot below, eash tag should contain 1 post as nested element.