Naiteek Vikamsey

Hello I have ZWO 071 MC-Pro camera which is roughly 3 years old. Recently when i took darks, i saw some horizontal pattern lines on it, on closer inspection i found that those lines are also present on my light frames. To which i inspected my old darks when the camera was new and there i couldn't see any such pattern, it was very clean and smooth dark file. So why recently such pattern developed? I tested by taking darks with different exposure, gain and offset but the pattern stays as it is. But my flat frames don't have any such pattern. Check the attachments below containing my dark, light, flat and dark flat files