SDL2 won't play with more than 6 audio channels

Question

I am trying to stream (raw) video and audio from a capture device as part of my home media setup (with my pc acting similarly to a receiver in a typical home theatre setup), but the biggest problem I haven't been able to get past is that I haven't been able to get ffplay (using SDL2 as its audio backend) to work with all 8 channels in 7.1 streams - two simply get dropped, despite it recognising 8 channel input or me specifying a 7.1 layout.

I have been able to confirm that all 8 channels are present in the source by first using ffmpeg to save the output of a speaker test to a file and playing that back with both mplayer (which works) and ffplay (which doesn't). I also wrote some minimal code to play the audio directly through SDL's API with the same result, so it's not the fault of ffplay. I might simply use mplayer if it weren't for the fact that piping output from ffmpeg adds too much latency for real-time use. I am using libSDL version 2.0.12 and ffplay 4.2.3, both of which are the latest at the time of writing and are ostensibly supposed to support 7.1 audio.

Using output recorded from speaker-test -c 8, I am using the following to play it back in mplayer:

mplayer -channels 8 -rawaudio channels=8 -format s16le -demuxer rawaudio speaker-test.pcm

and the following to play it back in ffplay:

ffplay -f s16le -ac 8 -af 'channelmap=channel_layout=7.1' speaker-test.pcm

No matter what I try, the two side channels get dropped. I couldn't figure out how to play raw pcm in SDL, so I repeated the same tests with wav output and used the following code to play it back:

#include <SDL2/SDL.h>

int main(int argc, char **argv) {
    SDL_Init(SDL_INIT_AUDIO);
    SDL_AudioSpec wavSpec;
    Uint32 wavLength;
    Uint8 *wavBuffer;
    SDL_LoadWAV("speaker-test.wav", &wavSpec, &wavBuffer, &wavLength);
    SDL_AudioDeviceID deviceID = SDL_OpenAudioDevice(NULL, 0, &wavSpec, NULL, 0);
    SDL_QueueAudio(deviceID, wavBuffer, wavLength);
    SDL_PauseAudioDevice(deviceID, 0);
    SDL_Delay(30000);
    SDL_CloseAudioDevice(deviceID);
    SDL_FreeWAV(wavBuffer);
    SDL_Quit();
    return 0;
}

The above code exhibits the same behaviour of dropping the two additional side channels, despite it being the latest version of SDL that should have supported 7.1 for many releases now. Why might this be happening, and how might I fix it?