render json returns different response passing literal value vs variable

I have this code in the controller:

    render json: item.item_tags.map do |item_tag|      {        things_count: item_tag.tag.things.count,        other_count: item_tag.tag.other_association.count,        tag: {          id: item_tag.tag.id,          name: item_tag.tag.name,          slug: item_tag.tag.slug,          things: ThingSerializer.new.index(item_tag.tag.things),        }      }    end

ThingSerializer here also has a map which goes through and serializes all the passed things.

It's supposed to return a list based on the map value.However, it returns a list of default item_tags, i.e.:

[{"id": 236,"status": "draft","position": 0,"item_id": 234,"tag_id": 123,"inserted_at": "2024-03-19T10:23:56.000+03:00","updated_at": "2024-03-19T10:23:56.000+03:00"}]

But if I do this:

    actual_json = item.item_tags.map do |item_tag|      {        tags_count: item_tag.tag.things.count,        other_count: item_tag.tag.other_association.count,        tag: {          id: item_tag.tag.id,          name: item_tag.tag.name,          slug: item_tag.tag.slug,          things: ThingSerializer.new.index(item_tag.tag.things),        }      }    end    render json: actual_json

It works as expected.

Why is it this way?

PS. I do similar things in other controllers and it works as expected. Only here it's not returning the expected value.