Mosaic Unknown Tile Blues

Plone Support Customizing Plone
,
1

Plone 5.1rc2
plone.app.mosaic 2.0rc8

I have a Mosaic page which has ended up with a number of tiles which can be relocated, but not deleted or edited.
The tag says "Unknown" when the pointer is hovering over the tile. Clicking the tile does not select it, so there's no way to change its format, etc. These were originally Image or text tiles.
How do we get at this stuff when it breaks like this?
If I change the view to "Document" the content in these tiles is no longer visible at all.
I tried saving a new 'test' layout, and the code looks like

 <body>
    <div data-panel="content"         data-max-columns="4">
      <div class="mosaic-grid-row">
        <div class="mosaic-grid-cell mosaic-width-half mosaic-position-leftmost">
          <div class="movable removable mosaic-tile mosaic-IDublinCore-title-tile">
          <div class="mosaic-tile-content">
          <div data-tile="./@@plone.app.standardtiles.field?field=IDublinCore-title"></div>
          </div>
          </div>
          <div class="movable removable mosaic-tile mosaic-plone.app.standardtiles.image-tile mosaic-tile-padding-right ">
          <div class="mosaic-tile-content">
          <div data-tile="./@@plone.app.standardtiles.image/51aeef6e35ca4270a4a70be71fc07972?_layouteditor=true&amp;&content=%3Cimg%20alt=%22Eclipse%20Enso%20August%202017%20Peter%20Fraterdeus%22%20title=%22Eclipse%20Enso%20August%202017%20Peter%20Fraterdeus%22%20src=%22http://www.makeamindfulmark.com/copy_of_2018-retreat-at-sinsinawa-mound/@@plone.app.standardtiles.image/51aeef6e35ca4270a4a70be71fc07972/@@images/79b203b5-8b6e-49be-a656-162b0b54e1d6.jpeg%22%20width=%22759%22%3E%0A"></div>
          </div>
          </div>
          <div class="movable removable mosaic-tile mosaic-IDublinCore-description-tile mosaic-tile-padding-right">
          <div class="mosaic-tile-content">
          <div data-tile="./@@plone.app.standardtiles.field?field=IDublinCore-description"></div>
          </div>
          </div>
          <div class="movable removable mosaic-tile mosaic-plone.app.standardtiles.rawhtml-tile mosaic-tile-padding-right ">
          <div class="mosaic-tile-content">
          <div data-tile="./@@plone.app.standardtiles.rawhtml/d65c1527-60f3-4fb9-81f0-724e6bcd3754?_layouteditor=true&amp;&content=%3Cp%3E%3C/p%3E%0A"></div>
          </div>
          </div>
          <div class="movable removable mosaic-tile mosaic-plone.app.standardtiles.rawhtml-tile mosaic-tile-padding-right ">
          <div class="mosaic-tile-content">
          <div data-tile="./@@plone.app.standardtiles.rawhtml/5ca7dd58-4b68-4d63-b96b-9a7460e20fb2?_layouteditor=true&amp;&content=%3Cp%3E%3C/p%3E%0A"></div>
          </div>
          </div>
          <div class="movable removable mosaic-tile mosaic-plone.app.standardtiles.image-tile ">
          <div class="mosaic-tile-content">
          <div data-tile="./@@plone.app.standardtiles.image/0c18264e17a84b4ea5caa7bfbe636378?_layouteditor=true&amp;&content=%0A"></div>
          </div>
          </div>
          ...
          </div>
          </div>
        </div>