Placement of feedback line on DC DC converter, before or after cap? (and dropout problem with LT8390)
From their reference schematics



it is clearly seen that the feedback wire (R18) comes from the main filter cap C20, and not from the point of load. It is actually in between two caps, C19 and C20. 

More, your entire layout has no resemblance with the suggested demo board:



All high-current loops in your layout have skinny traces instead of being solid rectangle of copper. I am afraid this is where your output ripple problem comes from.