Check the Apple serial number sticker on top of the ram contacts. It’s a layered foil sticker and on mine the sticker seemed to have shifted or degraded so that the foil was shorting the edge contacts for the bottom ram slot. I removed the sticker and placed a new layer of kapton tape in between it and the contacts and the problem was solved.
I had this same issue happen with my mid 2012 macbook pro a1286. The issue turned out to be the apple serial number sticker on top of the ram slot bracket. For whatever reason, this is a layered sticker with foil, and I guess that the sticker had maybe shifted or degraded slightly with time and heat and shorted the ram contacts. I ended up removing the sticker entirely, laying down some heat resistant Kapton tape in its place, and putting the serial number sticker back on top of that to be sure there was no chance of the sticker shorting the contacts again. Problem solved.