ALMOOOST!
It did work but I don't understand why it worked on some pages and not on others. So when using this method, its worked completely fine when the watermark is applied to visible page. However, when the watermark is applied only to printed pages, some of those pages lose the water mark completely on print during the flatten/change blend space. I cant tell whats special about those pages that it works that way.
Maybe it will be just easier to use the "Keep position and size of watermark text constant when printing on different page sizes" option when adding the watermark and then everything looks evenly perfect(just need to manually center the watermark position for each file). Curiously though, any idea what could be the correlation between the "Keep position and size of watermark text constant when printing on different page sizes" and the opacity of page when adding an watermark?