Temp fix
I've been having this same problem for several months. I can't really say when it started or what caused it, but here's a temp workaround that works for me.
I view all of my email accounts in one folder (folder called "All Inboxes"). When I get the 'No sender, No subject" emails, all I do is hit the "Mailboxes" button at the top/left of the inbox screen. That takes me back to the Inbox root for all email account inboxes. Then I choose "All Inboxes" again and when I go back all the emails that had "No sender, No Subject" now appear normally with Sender, subject, message, etc...
It's just a temp fix, though, as usually later on I get more emails with missing info. Not a cure, but a quick temp fix. Hope it helps someone.