Dennis, my user is also using a Samsung Galaxy S5. I cant imagine that would have anything to do with it but it might. He is set up on a POP account but it was doing it with an IMAP as well.
Carlo I also have a user that routes everything (multiple email address) through gmail. He uses it like outlook, because its more compatible w/ some other software he has. And this has caused some issues as well for him but not the issue with emails being marked as "read".
I have not figured out a solution to this.