That's likely to be a poppler bug (poppler is the library used by Okular for pdf rendering).  Can you try to reproduce the problem with the pdftocairo tool?  If that shows the error too, then please open a bug report in the poppler bug tracker, and post the link here for future reference.

Thanks for the quick answer.
pdftocairo -pdf y.pdf x.pdf 
produces a file x.pdf which shows the correct result once opened with okular wheras y.pdf shows the wrong result

(In reply to Ferdinand Gassauer from comment #2)
> Thanks for the quick answer.
> pdftocairo -pdf y.pdf x.pdf 
> produces a file x.pdf which shows the correct result once opened with okular
> wheras y.pdf shows the wrong result
This is related to https://bugs.kde.org/show_bug.cgi?id=401999.

Okular currently works so that it draws a stamp icon *on top of* a page rendered by poppler. The icon gets selected by annotation meta data (probably /Name = Draft, in your case). The approach causes trouble if a PDF contains a pregenerated appearance for the stamp along with its meta data. The appearance is drawn by poppler, and the icon is drawn by Okular on top of it.

Albert already suggested a solution which should work here too:
> Actually fixing this pdf "the right way" is not hard,
> we just need https://paste.kde.org/pk6wzgiqs in poppler
> and then a "backendPaintsStamp" or similar in the
> okular core annotation.

(In reply to Tobias Deiminger from comment #3)
> Albert already suggested a solution which should work here too:
> > Actually fixing this pdf "the right way" is not hard,
> > we just need https://paste.kde.org/pk6wzgiqs in poppler
> > and then a "backendPaintsStamp" or similar in the
> > okular core annotation.
Did that ever get into Popper?

Don't think so, but honestly don't remember what was there.