I actually have this same thing happening on all my systems, although it’s much more noticeable on 240p systems. I’ve used two OSSCs, an official pre-HDMI one and a knockoff HDMI one. Both have the same issue. Used the power supplies that came with each and swapped them too. I’ve bypassed my SCART switch and I got the same issue, so that’s not the part of the chain. Running everything through power conditioners.
I don’t want to start blaming the design of the OSSC because I haven’t heard of this being a widespread issue, it’s just weird I’d have it with two different models. Any ideas?