Well I would agree that it seems that it is when it is trying to send a batch.
At the top of the message it says: Found in Gmail All Mail Mailbox
Then the user info is displayed, Subject below that then message : This message has not been downloaded from the server.
Further comments from other users and their comments are there. Just skips thru, some a downloaded and several are not.
This doesn’t occur on my laptop just the iPad.
I've had this going on for a few weeks now.