After a few weeks of using El Capitan seemingly random messages started appearing blank. Viewing the messages in the Google web interface showed that they were not blank.
Restarting the app usually fixes it.
I deleted all my emails (by deleting mail folder ~/Library/Mail) and started again - and it was OK for about a week - and now it's started doing it again. Rebuilding mailboxes doesn't help.
I found links around the web to cases of this back in 2010/2011 - like this https://discussions.apple.com/thread/2364054
But it appeared to go unsolved.
Is this just a bug in El Capitan - and is there a solution?