I discovered a bug. When you use the highlight lines property for code blocks as shown here…
…the highlighting for the higher line numbers gets confused, as shown with lines 27, 39, 60 in the following screenshot.
I’ve reached out to WordPress support and will let you know how it goes. I’ll try to file a bug for this.
[UPDATE] A WordPress happiness engineer created the following issue, which does an excellent job of analyzing and describing the problem: https://github.com/Automattic/wp-calypso/issues/50577