Alright, the problem resumed today in compact mode. I switched to separate mode and enabled Always Show Tab Bar. The problem remains. *sigh*
Maybe that's another bug. I noticed once the tab bar didn't show even though I had it set to always show. I don't think I've seen that bug since restarting Safari after making that change.
Could you confirm that you actually see the tab bar? If you don't you can always force it to appear by adding a second tab (even just an empty one with command-t).
Super annoying for sure. But, having a way to work around the issue without having to paste the URL into a new window would be good. Pasting a URL often does not recreate the window in the state that you left it, even though it might appear to do that. Pages evolve thanks to code running in the web page, even if the URL doesn't change.