I assume your camera was in video mode when you send the command?
Can you please set camera to still image mode, and test if it works too?
Yes, it does work in Photo Mode too.
Delay between uart command or physical button press is simular.
It would be nice if somebody could analyse how cameras do (3D) sync via HERO bus cable.
I guess if HERO bus cable is connected between cameras, they enter some sort of special state where camera is waiting to start shooting.
Probably cameras will communicate via I2C or simply trigger one line on HERO bus.
Maybe 3D sync mode can be enabled with the right combination of ID pins in firmware v01.01.46?