I'm not suggesting that you are, only that you are mistaken.
Nothing but Sport Mode races, in Daily Races and FIA GT Online Championship races, affect either your DR or SR.
Some people swore blind in the game's early days that other things affected DR - some claimed that their DR fell if they didn't race for a couple of days - but they don’t.
My gt sport stats:
Everybody can see how my DR is dropped today (from 4919 (C) to 1755 (D)).