Devise remember_me! not working

Question

I have the following code in my sessions_controller.rb file:

module V1
  class Users::SessionsController < Devise::SessionsController

    # POST /sign_in
    def new
      user = User.find_by_email(params[:user][:email]) if params[:user] 
      if user && user.valid_password?(params[:user][:password])
        sign_in(:user, user)
        current_user.remember_me!
        head 200
      else
        render status: 200, json: {errors: ["Incorrect email / username or password."]}.as_json
      end
    end

    # ...

  end
end

The above code seems to work fine, however, after signing in and visiting my /posts route and I get an error telling me "You need to sign in or sign up before continuing."

The structure of my app:

controllers
├── v1
|   ├── users
|   |   ├── registrations_controller.rb
|   |   ├── sessions_controller.rb
|   |   ├── api_controller.rb
|   ├── posts_controller.rb
application_controller.rb

In my posts controller I have set before_action :authenticate_v1_user!. I'm using rails to build an API and I'm testing the above routes by sending requests from a tool called Postman. I've also tried testing from my AngularJS app (on a different subdomain than the API). Why would I get an error telling me to log in when I just logged in by sending the correct credentials to /sign_in. Are there special considerations since this is just an API?

EDIT:

My user.rb file:

class User < ActiveRecord::Base
  has_many :posts
  has_many :comments

  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable

  validates :email, presence: true, uniqueness: true
  validates :password, confirmation: true, presence: true
  validates :password_confirmation, presence: true
end

My routes.rb file:

Rails.application.routes.draw do
  devise_for :users, controllers: { sessions: "users/sessions", registrations: "users/registrations" }
  devise_scope :user do
    post "sign_in", to: "v1/users/sessions#new"
    delete "sign_out", to: "v1/users/sessions#destroy"
    post "register", to: "v1/users/registrations#create"
  end

  namespace :v1, defaults: {format: 'json'} do
    devise_for :users, controllers: { sessions: "users/sessions", registrations: "users/registrations" }
    # Routes for posts
    get 'posts(/index)', :to => 'posts#index'
    post 'posts/create'
    delete 'posts/:id', :to => 'posts#destroy'
    get 'posts/show'
    put 'posts/:id', :to => 'posts#update'
  end
end