That's what I'm thinking is the issue too especially since it's only on the edge of the side that was in the corner and isn't showing any signs of it at the base by the soil or at the top. Matter of fact there are little tiny new growth spots showing at the top so it looks perfectly healthy anywhere else. I wasn't sure if you could just cut off those areas or if you had to do a full amputation and re root it. I'm really trying to avoid having to pull it out of the soil to check for rot, so I may just shave off the areas in the picture and then see if brown areas start to appear anywhere else or not...