Wasted your time there, didn't mean to. Noticed this in the QMK docs Some controllers (e.g. Blackpill with DFU compatible bootloader) will need to be flashed with handedness bootloader parameter every time because it is not retained between flashes. I understand that you define it by MASTER_LEFT because you have connected the USB on the left half, and that "notice" text is relevant to another way of defining the handedness. But maybe it's a specific Blackpill thing.

Never worked with keyboard firmware but done my share of editing config files before maybe you should remove the # In front of define left / right.