When I leave a single empty line after a header, container, or paragraph, it turns into two empty lines when viewed on my world (ie, the public presentation).
When I inspect the html of the page, it looks like it adds the empty line as both a br linebreak, and a span set to display as a block.
Steps to Reproduce
Make an article in euclid in the new athena editor
Write a header, container (potentially any element that creates a block)
Leave one empty line, and then write some text afterwards