TYPO3 v8.5 with Fluidtemplate and fluid-styled-content - Partials not working

I use TYPO3 v8.5 with fluid-templating (with layout, partials and template) and fluid-styled-content. However, as soon as I create a fsc-element e.g. text&media on my site, the partial files are ignored and only the section from the template file is output in the FE. If I set the content-element to "hide", then the correct site-structure with partials were output in the FE.

My root-template is:
page = PAGE
page.includeCSS {
  main = fileadmin/Demo/Resources/Public/Css/main.css
}
page.10 = FLUIDTEMPLATE
page.10 {
    layoutRootPaths {
        10 = fileadmin/Demo/Resources/Private/Layouts
        20 = fileadmin/Demo/Individual/Private/Layouts
    }
    partialRootPaths {
        10 = fileadmin/Demo/Resources/Private/Partials
        20 = fileadmin/Demo/Individual/Private/Partials
    }
    templateRootPaths {
        10 = fileadmin/Demo/Resources/Private/Templates
        20 = fileadmin/Demo/Individual/Private/Templates
    }
    variables {
        contentMain < styles.content.get
    }
    file.stdWrap.cObject = CASE
    file.stdWrap.cObject {
        key.data = pagelayout
        default = TEXT
        default.value = fileadmin/Demo/Resources/Private/Templates/DefaultTemplate.html
        pagets__1 < .default
    }
}

My page-TSconfig:
mod.web_layout.BackendLayouts {
    1 {
        title = Default Template
        config {
            backend_layout {
                colCount = 1
                rowCount = 1
                rows {
                    1 {
                        columns {
                            1 {
                                name = Main Content
                                colPos = 0
                            }
                        }
                    }
                }
            }
        }
    }
}

DefaultLayout.html :
<f:render partial="Header"/>
<f:render section="Main"/>
<f:render partial="Footer"/>


Footer.html :
<div class="footer-container">
  <footer class="wrapper">
    Hier steht der Footer-Text
  </footer>
</div>


Header.html :
<div class="header-container">
  <header class="wrapper clearfix">
    <h1 class="title">LOGO</h1>
    <nav>
      Hier erscheint die Navigation
    </nav>
  </header>
</div>


DefaultTemplate.html :
<f:layout name="DefaultLayout"/>
<f:section name="Main">
<div class="main-container">
  <div class="main wrapper clearfix">
    <article>
      <f:format.raw>{contentMain}</f:format.raw>
    </article>
  </div>
</div>
</f:section>

Does anyone know this behavior and can give me a tip what's wrong with my code. In TYPO3 v7.6 this code works fine.

There is currently an open bug (see https://forge.typo3.org/issues/77235) regarding this issue. Try to rename your Partials as a workaround until this is fixed.