After some months of use, I noticed that the 60/f2.8 micro changes aperture while focusing. I work on A (aperture preferred) and set to 2.8 but as I focus the aperture changes to higher number and when I dial back, I could not reach the 2.8. Had to switch the camera off, take off the lens and snap it again and turn on the camera to get the aperture. Went to the shop where I purchased the lens, they sent it back to Nikon service. After a month, it came back with the same problem. Is this normal or is there something wrong? Thanks for your opinion.