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Screenshots look fine to me...theres no marks or spots on them.
Can't you take a photo of the screen..from a camera, or someone elses phone?
Aswhat you are seeing are marks as a part of the screen, as a screenshot is only what is shown on the screen, not of the screen.
Screen shots won't show defects or issues in the actual display panel as they are just images from the system capturing it.
What you need, is a photo from another device that shows the panel showing a white or grey full background
I agree with the above, so If there's nothing on the screen from the outside, I found an article with what I think would be the same issue you have, here check it out anyway.
https://www.sony.co.uk/electronics/support/mobile-phones-tablets-mobile-phones/articles/SX116801
Tnx for your answers, i Just checked all the screenshots i made, and the stains are gone now...i shall wait for it to happen again and then make photo with another phone. Yesterday i tried all the tips and tricks except for factory reset. This was the second time i had this problem , previous time was also after long hours of use. I'm sure it will happen again...Tnx for now.