Known LB issue to me happens once in a while with MBPR at my repair shop. Check out at LB keyboard connector port. You will notice corrosion or kind of stuff most probably internal condensation impact due to moisture trapped at some point of time. User will never realize as liquid spill - understandable. If you have a magnify glass, you can use that to look into keeping it vertical (I assume you have taken apart). If you do not see anything, use 100% alcohol at the tip of a soft brush and try to clean the LB connector port to Keyboard. After cleaning, dry it with a blower (hair-drier works if you blow with cool air) and assemble the topcase w/h keyboard. See if it resolves. If not, change logic board. Because the connector port is failing at one line.