Honestly it wasn’t even until recently that I started hearing about TTL sync, so I’m not even sure what my cable is. I did buy it a like the year before.
For a stock Super Nintendo, the RGB cable should have a resistor on the sync line, that’s the yellow wire on your picture. Unless that resistor is hidden in the other end of the cable (unlikely) your cable doesn’t have it.
The most likely reason it doesn’t have it is because this issue wasn’t widely known about until recently. The standard TTL sync signal will work on most equipment, though it’s not recommended because the voltage is higher than spec. The fact that it’s producing some weird results with your OSSC isn’t terribly surprising since the signal is not to spec so could be causing any number of issues.
Your Voultar board is set to output TTL sync, the same as a regular Super Nintendo, so really you need that resistor on the sync line.
Your options are:-
1) Reconfigure your Voultar mod board to output 75 Ohm sync rather than TTL sync. Some folks argue this is best since the less components in your SCART cable the better.
2) Order a newer SNES SCART cable from Retrogamingcables.co.uk, or add the resistor to your existing cable yourself.