Just an update on my case...looks like I'll have to either send my ASI2600MC to Agena Astro to get the sensor cleaned or I'll need to do it my self. It could be a quick fix especially if I clean it myself. However, just read today in ZWO Camera Facebook group that someone else(and several others with ASI2600MC/ASI1600MM) all had similar issues. Cleaning the sensor seems simple enough but it does not address the root cause. It's very difficult for end users to speculate what might have caused this ugly oily smudge on the sensor. (Shooting flats is not the answer. The one example shown in the Facebook post covered large portion of the sensor.)
Users are guessing if it was the storage temperature, the position of the camera, the anti-dew heater, solder flux....what if the issue happens again after sensor is cleaned? We all love ZWO products and ASI2600MC had been my favorite OSC since I pre-ordered it when it came out. We truly hope ZWO can take this issue seriously and come up with a permanent fix or improved design to address this issue.