I did some reading on internet and it seems my M100/4 has some haze in it on the inside of front element, which effect the contrast. Clearly visible when lit with flashlight from the rear. I have a feeling it's gotten worse from June 2018 when I shot this lens in SIC in APSC format. I remember it has always been bit on low contrast, but don't remember the los of constrast been this drastic when pointed towards the sun.
Here a shot from previous round for comparison:
Well, forecast for next 10 days doesn't offer any sun, so maybe I'm good for this month