Rails: Validation-errors on object are gone after render :new

I have implemented following validation on my model-class:

class Department < ApplicationRecord
  has_many :employees, dependent: :destroy

  validates :name, presence: true, length: { minimum: 3, maximum: 50 }
end

It works fine. It isn't possible anymore to create Department-objects without providing a name.

But in my template there are no errors available.

Error-Messages: <%= @department.errors.full_messages.size %>
<% if @department.errors.any? %>
  <% @department.errors.full_messages.each do |message|%>
     <h3><%= message %> </h3>
  <% end %>
<% end %>
<%= render "form", department: @department %>

The error-messages debug-output shows always 0.

My controller-code:

   def new
     @department = Department.new
   end

  def create
    @department = Department.create(department_params)

    if @department.save
      puts "-- Positive --"
      redirect_to departments_path
    else
      puts "-- Negative --"
      puts @department
      puts @department.errors.full_messages
      puts "Error message: #{@department.errors.full_messages.size}"
      puts " ------------ "
      render :new
    end
  end

Output:

-- Negative --
#<Department:0x0000000109b59398>
Name can't be blank
Name is too short (minimum is 3 characters)
Error message: 2
 ------------ 

Means: In the controller is everything as expected. But in the view-template errors is empty (size 0).

What goes wrong here? How can it be fixed?