I also have this issue. Only way to fix is to change to another input and then go back to input1 (RGBS – AV1). This issue does not occur on PS1 games on PS2, only on PS1 so is not game specific.
The Sony intial boot up logo causes the shimmer and then once its gone if you switch inputs it clears the problem.
If you turn the OSSC on after the Sony logo has disappeared you don’t get this issue.
I’ve had a playabout with the settings but nothing fixes other then switching to another input and switching back.