I've reproduced the problem again. Here are revised steps for trying to duplicate the issue. Seems like it works if I do exactly what I wrote before. Sorry.

All without quotes.

Search for "blue roses"
Click on any result
Use BACK command in browser to return to results
Search box correctly reads "blue roses"
Type over the word "blue" with the word "red" and search
Click on any result
Use BACK command in browser to return to results
Google search string says "blue roses" and shows results for blue roses

Obviously "blue roses" is an example, it seems to happen regardless of what I'm searching for.