the Front Left Mono problem only affects Dolby Audio tracks on my setup, Atmos music seems to be ok.
the only way i've been able to get around it so far is by going into the ATV's Settings while a Dolby Audio tracks playing, change the format to "5.1" (which my AVR then shows as "Dolby Digital") then change to Stereo (Atmos is greyed out and Off while doing this toggling) then change format back again and Atmos is re-enabled and the channels are all back to normal.
its a faff, doesn't always work and sometimes i've had to do it several times during the course of an album! Apple need to sort it out - now!