Ok - l looked at the raw email, and where the HTML version is missing spaces, the text version has the correct spaces. However, the text version is missing other space characters. There’s no rhyme or reason to it.
Maybe it could be a character-encoding glitch in the course of copy/pasting the affected topics from a legacy platform…? EDIT: Nope. It’s continuing with current posts, and also with other emails—not just the summary.
More recent Discourse summaries featuring current posts are not having the same issue, so I won’t worry much unless I see it continue. EDIT: It’s continuing.
(Side note: just to watch for this stuff, I now wish I could force a comprehensive summary sent to my admin account on a daily basis – regardless of me being constantly logged in.)