My Samsung 75 4K TV model :UE75TU8000, while on eARC mode: Auto, on port HDMI2, is hooked with an HDMI2.1 eARC approved cable, to an AV Receiver ( Yamaha RX-A2A, with eARC capabilities ) at an eARC's HDMI port.
Running Yamaha's MusicCast App, the status line indicates, for a brief instant, Digital TrueHD ( Which indicates protocol handshake negotiation), but then it defaults to PCM 48KHz, with no surround and no Dolby of any kind!
However, If I disable the eARC feature at the TV ( i.e. ARC mode) , I get ARC behavior i.e. status line displays Dolby Digital Plus / 48KHz, with surround.