Adding the .Net Core renderer to an existing .Net MVC solution and site. In .Net Core page layout and template, there are multiple placeholders, marked with the data-sfcontainer="" HTML property. In the template and page editor, the placeholders are properly and can add content to them. However, after saving and/or publishing, the content disappears from the placeholder that are added to it and drops to outside of the page template, with the so far sole exception that content placed in the data-sfcontainer="Main" placeholder is appearing properly.  It is observed that the content in the [sf_object_data] table and the placeholder names appear proper.

The template in question is a recreation of the .Net MVC template and uses several placeholders, including inside partial views for the header and footer. The multiple placeholders have been tested in the main layout as well as the header and footer partial layouts with the same behavior presenting in all placeholder containers except Main. The simplified cshtml file body looks like the following:

<body>
 <div class="sfPublicWrapper" id="PublicWrapper" style="overflow-x: hidden;">
  <top data-sfcontainer="top">@(renderContext.IsEdit ? "Top" : "")</top>
  <Alerts data-sfcontainer="AlertsContainer">@(renderContext.IsEdit ? "Alerts":"")</Alerts>
  @await Html.PartialAsync("TemplateSections/_TrackingConsentDialog")
  @await Html.PartialAsync("TemplateSections/_header")
  <div id="content" role="main">
   <main data-sfcontainer="Main">@(renderContext.IsEdit ? "Main":"")</main>
  </div>
  @await Html.PartialAsync("TemplateSections/_footer", siteConfig)
  <bottom data-sfcontainer="Bottom">@(renderContext.IsEdit ? "Bottom":"")</bottom>
 </div>
 @RenderSection("Scripts")
</body>

As to the <main> tag, it is a valid HTML element which is why only the content in the "Main" placeholder is rendered correctly.

The use of non-standard HTML elements such as <top>, <bottom> and <Alerts> in the razor view template.  As Sitefinity does not recognize these tags, it caused the placeholders not to bind correctly and the saved contents are dropped outside the page template.

1. Replace the element <top> and <bottom> with <header> and <footer> elements. 

2. Use valid tags like <div> with the appropriate data-sfcontainer attributes in it. For example:

<div class="site-wrapper off-canvas-wrapper">
   <header class="site-header" role="banner" data-sfcontainer="header">
   </header>
   <main class="main-contents" id="main-contents" style="margin-top: auto;">
   @if (this.renderContext.IsEdit || lastSegment == "news-test")
   {
      <div data-sfcontainer="bannerWidget" id="banner-widget">
      </div>
   }
   </main>
</div>