I’d be interested to know how that cable is wired. The challenge with MD/Genesis is that C-sync available from DIN connector comes from VDP and is more or less weak (apparently depending on console model) if connected to normal 75ohm terminated input (TV, AV1 on OSSC etc.) without buffering circuitry. The cable could have LM1881 sync stripper generating c-sync from composite video, it could be directly wired to DIN c-sync via a capacitor, it might have a buffer on this c-sync line, or some combination of previous. They should all work fine with OSSC AV1 if the console works on TV, but depending on the cable implementation the sync signal may actually be quite bad and needs some options tweaked on OSSC side (try “analog sync LPF” first”).
There is actually an additional setting on the digitizer chip which *might* improve syncing of problematic systems – it’d help if I had such system to try and debug, but I can release a test firmware with that option exposed if no other solution is found.