How do I prevent Safari's untitled window from reappearing on MacBook Pro?

Every day after a couple of minutes of working in Safari, I'm getting this Untitled window in Safari's Dock windows switcher. I can't choose it or open it - it’s like minimized, it’s getting focus, but I can't see it even through Expose. The only way to close it is to quit all Safari instances. But it will reappear after a while. I noticed that it takes the Tab Group name and can change it if you change Tab Group in another Safari instance, its "Main - Untitled" on the attached screenshot. Do you have any ideas what might cause it?

[Re-Titled by Moderator]