In Calibre I usually select the deficient text and click on the "supprimer la mise en forme" icon in the comment box. Sometimes it didn't work and I understood why reading this thread.
Now, if it doesn't work, thanks to marp68'comment I go to the html Source.
And I just get rid of anything in between < and > except one <div> to begin and one <\div> to end. I prefer to add eventual breaks / bold / italic in the "vue normale" (possible translation "regular view").