Playback post bounce sounds mono?

[cabbagerock]

I have discovered a bug in core audio where whichever output is assigned to the stereo right side channel in Audio Midi Setup receives the right side audio 6 db down AND the same right side audio is also fed to the next adjacent output. So for example, if stereo LR are assigned to outputs 1 and 2 of an interface respectively, channel 1 outputs the left side audio normally and channels 2 and 3 output the right side audio at 6db down.

My work around is to map LR to different unused outputs in audio midi setup, close and reopen the app that is using core audio (Quicktime in your case?), and then remap to your desired outputs.

Not sure if this related to your problem but might be worth a shot. I am on Catalina using HDX core audio and it happens on any interface, haven't had a chance to narrow it down further.