Yeah it didn’t make much sense either. I don’t think the circuitry in the OSSC is an issue, as it works fine with my 4k TV. The hdmi input on my crt does work, I’ve used it with the gamecube gchd mk-II, and that’s worked fine. That’s also how I know it supports 480p through hdmi.
I’m guessing it is the TV struggling with nonstandard hz. Is there any way to get around that?