Text insets display incorrectly in HTML5

FrameMaker 2017 here, on Windows 10.

My entire document displays correctly in Frame, and it displays correctly when saved to PDF format.  But a text inset displays incorrectly when I publish to HTML5.  I've tracked the culprit down to text and/or graphic objects placed on top of an image inserted by reference.

To duplicate this, follow these steps:

1. Place a paragraph or two of text at the top of an empty FM document.
2. Insert by reference an image below the text.
3. Click Graphics > Tools and use the Draw a Line or Place a Text Frame buttons to place a graphic object on top of the image.
4. Save the FM document.
5. Insert that FM document as a text inset into a different FM document.
6. Publish the parent FM document to HTML5.

When I do this, the image I inserted in step 2 is replaced by an image of the text that was above it.  Not only this, but the font size and margins are smaller/narrower in the image.  If the image I inserted is taller than the paragraph of text, then the bottom half of the text image will be white space.

For reference, see these before/after images.  In the before image, the thick diagonal line is what I added using Graphics > Tools.  The other lines were made in Photoshop.  If I delete the line I made using FrameMaker, the text inset displays properly in HTML5 (when published as part of the overall document).

Of course, if I publish only the text inset to HTML5, it displays fine, as you can see.