Center Channel Speaker Conundrum

(Cross Post in Amps/Preamps, Home Theater, Digital, and Misc. Audio)  Conundrum for the group: I have a Marantz AV7005, probably seven or eight years old, which purports to resolve Dolby Digital Plus signals as one might receive through Netflix, Amazon Prime Video, and so forth.  Indeed, when playing such sources, the pictogram on the Marantz display screen reflects "Dolby Digital +," and the owner's manual lists DD+ as a supported, decodable, transmissible signal.  

Here is the problem — across different streaming devices (PS3, PS4, and now a new Roku unit) the center channel will not produce sound from Netflix or Prime Video programs.  Instead, the center channel signal apparently splits into the left and right front speakers (piggy-backing on the appropriate left and right front signals). 

Oddly enough, the Marantz’s display pictogram not only reflects the correct DD+ signal and codec, but all of the relevant speakers are illuminated as being recognized and fed, including the center channel (center, left and right fronts, left and right surrounds, and subwoofer are all shown as active).  Indeed, all of the other speaker channels play and sound fine, including the subwoofer. 

As for other potential contributors, the center speaker itself works fine, and when fed by the same AV7005 and amplifier connection, plays through multi channel stereo and other programs perfectly well.  Thus, I know both the speaker and amp channel operate correctly.  The speaker EQ is also set sensibly, with the center channel not suppressed unduly from a volume standpoint.  

So, does anyone else experience this?  Any thoughts on how to resolve this riddle? Thanks!