Oh no... I do not think you're being precious hun. I detest people who pass off their rudness as being shy.
Obviously your DSS has manners because he is trying to get her to say thank you, perhaps she is just rude.
Gee, I hope she snaps out of it soon before your DSS gets tired of being with someone so rude and decides someone who can say thank you makes a better girlfriend. Does she even thank him when he does something nice for her?