I didn't attach a sample because I'm not doing anything fancy here. The symptom is not particular to the input text.
As presented on this forum, both of our messages are valid Markdown, so you can cut and paste either message text, save it to a plain text file called x.md, drag that to Calibre, and click the "Convert books" button in the toolbar. It'll have the "Input format" set to "MD" due to the file name extension; set the "Output format" to EPUB if necessary.
When the conversion completes, the EPUB will take precedence over the MD, so just double-click the book entry in Calibre to view the EPUB version. Here's what I see in the Calibre E-Book Reader for the first page of my own rendered text:
So, wall-of-text, as I said.
EDIT: As for the Calibre version, the above was produced with the latest, 3.39.1, but it's not a new symptom in that version. I'm only posting now because I've given up trying to hack my way around it from the end user side.