Thanks, paulb_nl, increasing sync Vth (to at least 146 in my case) does indeed solve the issue, sync LPF was at 2.5 already. Could leaving sync Vth at this cause other issues? (Since it’s listed as a last resort option on the wiki.)
I’m using the standard cable that came with the SNES right now, though I ordered a luma sync cable recently. Could that solve the problem as well?