Broken in Rails 5.2.2

[thedarkside]

Hey,
im having an issue after upgrading from Rails 4.2 to Rails 5.2.2.

My setup:

class PagesController < ApplicationController
  caches_action :show, layout: false, expires_in: 1.day
  def show
    @page = Page.find_by(...)
    @rendered = render_to_string :inline => @page.content, :type => 'erb', :layout => false
  end
end

<%= raw @rendered %>

Explanation: It loads some ERB HTML from db and renders it.

After the upgrade to 5.2.2 the first cold response includes the contents of @rendered html-escaped which leads to an unpleasant result because the user sees the page as pure html text.

The log output looks like this:

I, [2019-03-26T14:46:44.352243 #10012]  INFO -- : Processing by PagesController#show as HTML
I, [2019-03-26T14:46:44.352300 #10012]  INFO -- :   Parameters: {"lang"=>"en", "permalink"=>"contact"}
I, [2019-03-26T14:46:44.354388 #10012]  INFO -- :   Rendering inline template
I, [2019-03-26T14:46:44.354847 #10012]  INFO -- :   Rendered inline template (0.4ms)
I, [2019-03-26T14:46:44.355110 #10012]  INFO -- :   Rendering pages/show.html.erb
I, [2019-03-26T14:46:44.355191 #10012]  INFO -- :   Rendered pages/show.html.erb (0.0ms)
I, [2019-03-26T14:46:44.356112 #10012]  INFO -- :   Rendering html template within layouts/application
I, [2019-03-26T14:46:44.356174 #10012]  INFO -- :   Rendered html template within layouts/application (0.0ms)

the following warm responses contain the unescaped @rendered content and the log looks like this (with fine results):

I, [2019-03-26T14:46:48.135881 #10012]  INFO -- : Processing by PagesController#show as HTML
I, [2019-03-26T14:46:48.135915 #10012]  INFO -- :   Parameters: {"lang"=>"en", "permalink"=>"contact"}
I, [2019-03-26T14:46:48.136769 #10012]  INFO -- :   Rendering html template within layouts/application
I, [2019-03-26T14:46:48.136827 #10012]  INFO -- :   Rendered html template within layouts/application (0.0ms)

And finally a log output with disabled action_cache (with fine results):

I, [2019-03-26T14:53:38.090452 #12603]  INFO -- : Processing by PagesController#show as HTML
I, [2019-03-26T14:53:38.090505 #12603]  INFO -- :   Parameters: {"lang"=>"en", "permalink"=>"contact"}
I, [2019-03-26T14:53:38.092181 #12603]  INFO -- :   Rendering inline template
I, [2019-03-26T14:53:38.095078 #12603]  INFO -- :   Rendered inline template (2.8ms)
I, [2019-03-26T14:53:38.095415 #12603]  INFO -- :   Rendering pages/show.html.erb within layouts/application
I, [2019-03-26T14:53:38.095500 #12603]  INFO -- :   Rendered pages/show.html.erb within layouts/application (0.0ms)