Apologies for the delay in following through - marking season...
I think I have located the problem and can reproduce it. Moodle logs didn't help (nothing out of the ordinary), but marking did. The students had to respond to an email as writing prompt and include basic headers in their reply. The prompt looked like this:
To: John Doe <email@example.com>
From: Jane Doe <firstname.lastname@example.org>
Subject: Lore ipsum
The replies of the stundents who complained (and sadly several more) were truncated:
To: Jane Doe
IMHO this points to a character escaping issue with "less" and "more." I found that the answer was completely truncated if there was no closing "waka" or the email address (if properly enclosed) disappeared. The reply format was plaintext (no HTML editor).
I hope this helps, and the issue can be confirmed and fixed.