Hi,
I have found the answer to my own question.
It appears that my problem was earlier in my pipeline than I thought. I am starting from a TXT file that I convert to a docx before attempting docx->epub with calibre. It appears that neither Word nor LibreOffice handled well the txt->docx conversion, i.e. they introduced line breaks after each line in the text. Then, of course, calibre kept these line breaks by adding multiple <p> elements.