Even though respective BRAVIAs support DD+/EAC3 encoded Atmos to be carried over HDMI-ARC, it does not work for Prime Video, probably due to Sony lacking of support for the dedicated EAC3 JOC API (ENCODING_E_AC3_JOC) which has been introduced with Android 9.0 Pie and would have to be backported to Android TV 8.0 Oreo. Other apps like Kodi use the ENCODING_E_AC3 API for Atmos.
Even though respective BRAVIAs support DD+/EAC3 encoded Atmos to be carried over HDMI-ARC, it does not work for Prime Video, probably due to Sony lacking of support for the dedicated EAC3 JOC API (ENCODING_E_AC3_JOC) which has been introduced with Android 9.0 Pie and would have to be backported to Android TV 8.0 Oreo. Other apps like Kodi use the ENCODING_E_AC3 API for Atmos.