I checked all my before pictures and found this issue happened for long time already. Just because I took picture on ISO 160 mostly so I don't found this issue before. Actually, I can see the the noise boundary from ISO 320 picture if I review the picture by 100%. And I checked the pictures I took in the first few months I purchased M8 then the pictures are fine.
I contacted with the local Leica dealer and told them the issue and what I found. They confirmed my M8 is broken and needed to send it back to Germany for repairing. Luck for me is my M8 is still under warranty, it will expire th