Why do some text messages show new after they have been read?

[AC Question]

I have a Samsung Galaxy s6 Edge. I've had it for about a year. Recently I'm having text messages show up as new when they have already been read. Anyone else have this issue. It seems to be from only one person. They have a iPhone 6 Plus. I have other friends with the same phone and it's not happening when I get their text messages.