Some cells black when converting DOCX with embedded XLSX tables to HTML

When using LibreOffice soffice to convert DOCX to HTML and there are embedded tables from XLSX in the source DOCX, some cells appear “blacked out.” Some tables convert OK, some tables have one or two cells affected, some tables have many. I cannot determine the cause of this issue and am looking for suggestions.

I’m using the command

soffice --headless --norestore --nofirststartwizard --nologo --convert-to html [some docx] --outdir [some dir]

Note that if I unzip the DOCX and convert all EMF files in word/media using

soffice --headless --norestore --nofirststartwizard --nologo --convert-to [jpg|gif|png] [some emf]

then converted images do not have blacked out regions. They look just fine.

Tested with LibreOffice on Ubuntu.

Any ideas?