Thanks. I played around with different values for the H-sync tolerance, but couldn’t get anything.
One thing I’ll add here is that the behavior in my original post is from the luma cable. When I use the c-sync cable, the OSSC display briefly flashes “AV1: RGBS 311p. 15.73kHz 50.50Hz” for maybe half a second, then switches to “no sync”. The screen does change from the gradient screen to a black screen. There is no sound at all with the c-sync cable.