After switching from erb to haml the view templates are duplicated on redirects

I am working on a small flashcards application as an exercise to get better with rails.
After I switched from .erb to haml as templating language, strange things happen when the application redirects with redirect_to.

1. The Browser History doesnt change, the URL in the address bar stays the same
2. Instead of re-rendering the whole page with the layout and the new view, the new view is simply inserted at the end of the DOM of the current page. So I see the layout + the old view + the new view. (But I dont want to see the old view)

Here is the code for the starting view:

# app/views/dashboards/show.html.erb
<h1>Dashboard</h1>

<h2>Cards ready for Training</h2>
<ul>
  <% @topics.each do |topic| %>
    <li><%= topic %>: <%= topic.cards_ready_for_training.count %> Cards ready for training <%= button_to "Start Training", trainings_path(training: { topic_id: topic.id }) %></li>
  <% end %>
</ul>

...

After the user clicks on "Start Training" the TrainingsController#create is executet.

# app/controllers/trainings_controller.rb  
def create
    @training = TrainingFactory.create_from_cards(cards_scope)

    if @training.persisted?
      training_progress = TrainingProgress.new(@training)
      redirect_to edit_training_card_url(training_progress.next_card), notice: "Training was successfully created."
    else
      render :new, status: :unprocessable_entity
    end
  end

the we redirect the request to TrainingCardsController#edit

# app/controllers/training_cards_controller.rb
 def edit
    load_training_progress
  end

which renders the edit.haml file:

# app/views/training_cards/edit.haml
%h3= "#{@training_progress.pending_cards_count} cards left"

%h2= @training_card.question

= render "form", training_card: @training_card

%br

%div
  = link_to "Back to Dashboard", root_path

But instead of rerendering the whole page, the edit.haml view is just inserted into the current page.

If I convert the edit.haml back to erb, everything works as expected.

I have also tried converting all view templates to haml, including layout.html.erb. But that didn't help either.

Also: I only get that problem on redirects with redirect_to, while the default rendering of views in get requests works fine.

Okay I found a solution to the problem, however I do not understand why this fixes the whole thing. Maybe someone with more experience has an idea and can describe the cause of the problem.

After removing Turbolinks from the application, redirection works fine again.