Update on above.
Set it up, Oppo HDMI set to auto at first, then forced bitstream. turned on Disney app, tried Capt. Marvel, audio was DD+ on AVR, Oppo shows only HDMI Front.
Then I tried another Disney movie, like Dr. Strange. Well, it reverted to Prologic. Then I tried Capt. Marvel again, no DD+ but Prologic. Very curious.