I’m fairly certain Voultar sets his boards by default to attenuated 75Ω CSync.
I’d be willing to bet that the cable you’re using, also has attenuation built in (resistor on sync line), in order to bring the SNES TTL CSync output level down to 75Ω terminations.
So if these two are the case, you might be attenuating the sync signal twice, which could explain your issues? Voultar told me that if you’re using his board with 75Ω jumper on, then make sure there’s no resistor on the sync line in the SCART head on the cable.
Hope this helps