Thinking about it – is it possible to disable audio in over SCART? Maybe it’s still active when I’m testing audio signal over separate cable? Perhaps some interference in the SCART cable is still being picked up by OSSC and mixed with clean signal from the minijack in?
Or is the SCART audio disabled when minijack is in use?
Still that does not explain why the issue is present only with OSSC. Maybe it’s sensitive to this specific interference or the interference generated by OSSC’s power supply?