I'm hoping for some troubleshooting tips. I have a self-build split keyboard using Blackpills.
If I flash either side with 'SPLIT_KEYBOARD = no' in my rules.mk, then both sides work fine (but both working as the left side). If however I set 'SPLIT_KEYBOARD = yes', then both sides stop working.
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.