can't get fallbacks to work in rails + mobility

Question

I'm having some issues with configuring the mobility gem.

If a translation isn't present, I would like to get the default translation (in this case :de should fallback to :en). I built a test which should explain what I'm struggling with. I guess it's a misconfiguration, hopefully someone can point out what I'm missing here.

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  gem "rails", "~> 5.2.0"
  gem "pg", "~> 0.12"
  gem "mobility", "~> 1.2.9"
  gem "i18n", "< 1.9.0"
end

require "active_record"
require "minitest/autorun"
require "mobility"

Mobility.configure do
  plugins do
    backend :key_value
    active_record
    reader
    writer
    column_fallback true
    fallbacks({ :en => :de })     # this is the line that's giving me headaches 
    locale_accessors [:en, :de]
  end
end

I18n.available_locales = [:en, :de]
I18n.default_locale = :en

ActiveRecord::Base.establish_connection(adapter: "postgresql", host: "localhost", database: "testing", user: "postgres", password: "password")

ActiveRecord::Schema.define do
  #enable_extension "plpgsql"
  create_table "mobility_string_translations", force: :cascade do |t|
    t.string "locale", null: false
    t.string "key", null: false
    t.string "value"
    t.string "translatable_type"
    t.bigint "translatable_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["translatable_id", "translatable_type", "key"], name: "index_mobility_string_translations_on_translatable_attribute"
    t.index ["translatable_id", "translatable_type", "locale", "key"], name: "index_mobility_string_translations_on_keys", unique: true
    t.index ["translatable_type", "key", "value", "locale"], name: "index_mobility_string_translations_on_query_keys"
  end

  create_table "mobility_text_translations", force: :cascade do |t|
    t.string "locale", null: false
    t.string "key", null: false
    t.text "value"
    t.string "translatable_type"
    t.bigint "translatable_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["translatable_id", "translatable_type", "key"], name: "index_mobility_text_translations_on_translatable_attribute"
    t.index ["translatable_id", "translatable_type", "locale", "key"], name: "index_mobility_text_translations_on_keys", unique: true
  end

  create_table "contents", id: :serial, force: :cascade do |t|
    t.string "key"
    t.text "body"
  end
end

class Content < ActiveRecord::Base
  self.table_name = "contents"
  extend Mobility
  translates :key, type: :string
  translates :body, type: :text
end

class BugTest < ActiveSupport::TestCase
  test "should have fallbacks enabled" do
    model = Content.create!
    model.key = "foo"
    model.body = "bar"
    model.save
    model.reload


    assert model.key                 # works as expected
    assert model.body                # works as expected
    assert model.key(locale: :de)    # fails. expecting "foo"
    assert model.body(locale: :de)   # fails. expecting "bar"
    assert model.key_de              # fails
    assert model.body_de             # fails
  end
end