Loss of details and blurry photos are usually the culprit of a dirty camera lens. That there is the first and foremost answer anyone is going to give you. As for the second reason there is a known issue with the camera lens coating on the M8 that can cause a the same issue with night or day photos. This issue has been discussed extensively in this thread:
HTC One M8 Camera Lens Cover Issue
I can tell you that I had the same issue with night photos and although my lens was clean, ended up being the camera lens issue. After cleaning the coating off with a lightly moistened microfiber cloth, I was able to get the clarity of the photos back in your before pictures.
I would think that wouldn't
have been the issue due to the fact that the before pictures were clear. If the lens cover had been the issue then the before pictures never would have been clear in the first place.
No, same situation after cleaning...I did factory reset too, but no - again low night photos...
From looking at your pictures I at first thought it might have to do with the angle you took the pictures since focus and brightness can quickly change from one angle to the next. Most of your before pictures look like they were taken from a position closer to the ground, and a more of a horizontal angle than the after ones that appear to be from a slightly more diagonal, higher up, ground facing view which is also why I thought the flowers appeared more orange in the before and pink in the after, but your last picture (unless you mixed up the order and labeling) is counter to those others in both color and angle. (Aside from weather and age effecting the flowers directly of course).
Have you checked to make sure you don't have a zoom feature active/enabled? (The factory reset would have likely eliminated that as a factor, but I thought it worth asking anyway). To confirm you didn't possibly use the default camera app in one case, and the Zoe camera app in the other did you? As a test try taking the pictures with both apps, and see if one happens to turn out better than the other. I know the Zoe app was updated at one time, but I don't know about the default camera app. Its possible one of the default settings was changed from one release to another.
My HTC One m8 is still on 4.4.2 (on the Verizon Network), so unfortunately I can't tell you if the change is related to the 4.4.3 update or not.
Perhaps if you would post all the settings that are selected in each of the camera settings areas while you are taking those pictures then people who feel they have clear night pictures can tell you where their settings differ, and you can try their settings to see if it fixes the issue for you.