Welcome to the Humminbird Side imaging Forums Stano.
You do look to have some type of Si sonar reflection as seen in the Di sonar image you posted but without seeing an Si View we will have to take you at your word that it is only in the left Si sonar data. Although more common, I don’t think that this is a sonar reflection from the jackplate as your transducer looks too low on the transom for that. Instead I think that it may be the water intake screen causing this. Is there some way that you can take it off just to test and see if the constant left SI sonar line goes away or not?