Have found the key on dead man, opened underwater chest, got soaked note and finished all of the hidden teasure quests for White Orchard. Quest log shows completed, but the solid white icon remains with a white circle around it and I cannot understand what is missing or is this a "bug". Again, I have completed the chain of quests required . Is this just a "bug" or did I miss something?

It's a bug.

The only way i was able to prevent that bug from happening is by picking up the Ostmurk for the main quest before looting the underwater chest.

It greyed out for me, but if it's listed as completed in the quest log I wouldn't bother too much.