My bug report as promised. The following code block from GitHub was already line numbered. After installing the theme component titled Discourse Code Block Line Numbers, the alternating blank lines were included.
The original posting is here. And the information came in via a simple URL. More on Sankey diagrams on Wikipedia EN for those interested! 
Clarification; by alternating blank lines I mean the fully blank lines without any text whatsoever. The earlier rendering was not spaced out in this manner.