If you click on "Multiple Recipients" in the message list in the sent items folder the usernames are missing.
I wasn't able to replicate this. Here on my forums or my localboards.
It definitely happens here but I suspect it might be bad data in the database that could be isolated here.
It works fine on my localhost too.
Thank you for your bug report.
This bug has been fixed in our internal code repository. Please note that the problem will not be fixed here until these forums are updated.
With regards,
MyBB Group