My setup for it is this:
SNES -> RCA cable -> RCA/SCART Adapter -> OSSC
That’s your problem. The OSSC only accepts RGB and YPbPr component inputs. Feeding composite video from that yellow plug into a SCART shim doesn’t do any conversion to RGB. You specifically need an RGB SCART cable for your PAL SNES.
The OSSC will accept composite video, which is why it reports it has valid sync at the expected dimensions and framerate.