Image has different quality in different places

This is typically what happens without proper color management.

Photoshop and Windows Photos are both fully color managed. Your web browser may not be, depending on how you prepare your file.

Color management has three components:

• an embedded color profile in the image (sRGB, Adobe RGB etc)
• a monitor profile at system level. This is ideally made by a calibrator. If not, you may get generic manufacturer profiles through Windows Update. These are surprisingly often defective
• a color managed application that reads both these profiles and converts from one into the other

The reason you see this, is that LCD displays usually have a native dip in the shadow response, causing shadows to appear darker then they are. Color management corrects and compensates for this so that the end result is linear.

Could you please indicate the exact spot on the image? The images have different dimensions and are saved in different file formats. I cannot compare them side by side.

Now confirmed it also loses quality when I uploaded it to this post.