Can you share with us how your schema looks like?

For me it is weird. I’ve just tried basing on your input with the latest Ecto 3.4.4

defmodule EmptyJson.Repo.Migrations.CreateUser do
  use Ecto.Migration

  def change do
    create table(:users) do
      add :jsonb_field, :jsonb, null: false
      add :json_field, :jsonb, null: false
      add :map_field, :map, null: false
    end
  end
end

The schema

defmodule EmptyJson.User do
  use Ecto.Schema

  schema "users" do
    field(:jsonb_field, {:array, :map})
    field(:json_field, {:array, :map})
    field(:map_field, {:array, :map})
  end
end

With the insert itself

%EmptyJson.User{id: 1, jsonb_field: [], json_field: [], map_field: []}
|> EmptyJson.Repo.insert()

And got the next log

INSERT INTO "users" ("id","json_field","jsonb_field","map_field") VALUES ($1,$2,$3,$4) [1, [], [], []]

If you print out the d\+ users in psql client then

                                                  Table "public.users"
   Column    |  Type  | Collation | Nullable |              Default              | Storage  | Stats target | Description 
-------------+--------+-----------+----------+-----------------------------------+----------+--------------+-------------
 id          | bigint |           | not null | nextval('users_id_seq'::regclass) | plain    |              | 
 jsonb_field | jsonb  |           | not null |                                   | extended |              | 
 json_field  | jsonb  |           | not null |                                   | extended |              | 
 map_field   | jsonb  |           | not null |                                   | extended |              |