The tooltip remains on top of each page I visit (and it's quite annoying), until I close the Opera window (or quit Opera). When I re-open it, the tooltip shows in a different position.
Sometimes they even show one on top of the other, as in this second photo.

I believe it has something to do with the position of the Opera icon in my dock (which is located on the left-side of the desktop) and the mouse pointer location when I click on it: usually the Opera icon is right on top of my dock and the result is as shown above, but when I tried to move it down in the dock, the tooltip showing is that of one (or more!) of the lower items in the Opera sidebar, as shown here.

When I open Opera from Spotlight or a similar service, the tooltips only show whenever my mouse pointer is on the far left side of the screen.
From what I understand, this issue might be related to the "expansion" effect of the new window and the mouse pointer location during this animation..?

Opera for Mac version 57.0.3098.106.