There’s absolutely no reason why using composite sync rather than separate sync would do that. I suspect a cable/adapter problem if that’s what you’re seeing.
I had purchased a Dreamcast SCART cable from retrogamingcables.co.uk, previously I had used the official HKT-8100 VGA adapter. When I complained about the brightness loss, I was told it was something to do with the resistors needed to lower the voltage. Just for comparison, here’s a capture test I made before returning it. These were made by feeding the Dreamcast 480p signal into the OSSC with an mClassic attached. First with the official VGA adapter and second with the SCART cable.
That I can’t tell you, I do have an Amiga so I’ll test it but I’ve not managed to secure one for myself yet, let alone any for resale!
I’m in no hurry to buy an Amiga computer, but the PAL systems I own include a PS1, PS2, SNES, N64, Saturn, AV-modified NES top loader, and an original Xbox. I own a GameCube and Wii too, but I have HDMI adapters for both. Can you check how well the RetroTINK 5X handles these using RGB SCART?
That’s another thing too I’ve read about the 5X, I heard its more geared for people who use the HD Retrovision component cables rather an SCART leads.
EDIT: I misread your post, sorry. I see you don’t own a RetroTINK 5X yet.