I think that the Roku doesn't deliver DD or DTS? Sorry I don't live in the dying empire, so I can't confirm that, but the SB can only report the data feed it is fed. Roku may only output 2CH non Dolby sound. From what you say (maybe, I can't be certain, says the grammar police) the TV is passing through whatever format it is fed. My soundbar doesn't have an HDMI input, only optical, and it reports the various formats that my Oppo sends it via optical.
Asus VivoPC HTPC Win10 i5 16GB RAM Intel graphics OR Oppo 105D>Panasonic P50ST60.
Audio: Linn MDSM/2>Linn 6100/D>Linn Aktiv Ninkas (Linn K600 and silver RCAs), OR Optical out from Oppo>Vizio S4251w-B4 5.1
Synology 215j, Asus AC88U router