email not visible in users mail client

[Mark Foley]

I have an odd issue. One user has an email in her Maildir/cur folder named:
1545229920.27374_0.mail:2,. She cannot see this message in her mail client (Thunderbird). All
other emails have 'S' and 'W' components to the name, e.g.
1488471573.M167365P19808.mail,S=41356,W=42118:2,RS, but this one does not. Would that somehow
make a difference in it being visible to the mail client?

Why would this message have been saved without the 'S' and 'W' bits?

In fact, there are two such message with this abbreviated file name, both from the same sender.
Is there possibly something about the message that affects naming?

Dovecot version 2.2.33.2

THX --Mark