As noted in edits above, this problem continues with current user-posted content, not just admin cut-&-pasted content — and is now observed with summary, user_replied, and user_posted emails.
MailerSend support confirmed the space characters are missing when they receive the request from Discourse — so it would seem the error is with Discourse generation of the email…?
FWIW, space characters are not missing when previewing a generated summary — only when they’re received as emails.
At the same time, I’m having this issue with summary emails, reported by others starting in February:
These repeated posts are present in the generated summary previews.
I’ve done a command-line update & rebuild to see if that would flush out any kinks, but it had no effect.
If these things aren’t happening to everyone who’s up-to-date on the tests-passed branch, what might I look into in my setup?