OK, a bit more playing with this and now I have a clue.
I keep my Google preferences set to open a click in a new window. If I click on several different results, the target pages re-use the same window.
Now in that window, if I use forward or back, the URL doesn't change. However, if I use that window itself to navigate to just one more page, then the address bar returns to proper functioning on all previous pages. So it looks like the bug is connected to target="_blank"