It actually happened to me once, with my Japanese SEGA Saturn. I was messing around with my scart switchers, as I couldn’t remember which port the RGB cable was plugged into, so I was swapping a bit. I thought it was due to what I was doing, and – as you already noted – changing channel fixed it immediately.
I’ll try and see if I can replicate it tonight, or tell you what I do when I boot up my Saturn, since I don’t recall it happening ever again, after that one time.
Edit – In case it can help, I’m not using an RGC cable with the Saturn, as mine got somehow damaged (still wondering how it could happen…) and stopped carrying audio to the right channel, so I went back to the one I previously bought from TheFoo83.