JSON (AJAX) update of ActionText fails on Rails 6.1 RC1

I have a basic web app with a form using ActionText. When changes are made, a have a Trix listener that calls my controller via AJAX (autosave!). The controller looks like this:

  def update
    respond_to do |format|
      if @model.update(model_params)
        format.json { render json: @model }
      else
        format.json { render json: @model.errors, status: :unprocessable_entity }
      end
    end
  end

This was working absolutely fine on Rails 6.1 alpha, but since the update to Rails 6.1 RC1 the same code results in this error:

ActionView::MissingTemplate (Missing partial action_text/content/_layout with 
{:locale=>[:en], :formats=>[:json], :variants=>[], :handlers=>[:raw, :erb, :html, :builder, :ruby]}. 

Is anyone able to shed any light on why this might be? I have tried digging through the source but I'm none the wiser. I wondered also whether it was related to this issue.

Thanks in advance.

I've solved this problem as follows. Originally I had format: :json in the form_with declaration:

<%= form_with(model: @model, format: :json) do |form| %>

This was working in Rails 6.1 Alpha but not in RC1. Removing the format from the code solved the issue.

<%= form_with(model: @model) do |form| %>

The POST request now comes through as JS not JSON and this makes the problem go away.

Processing by MyController#update as JS