If you run :params{test:1} in 1 pane then run RETURN $test, you get 1 as expected. If you go back to the first pane (not a new query), change it to :params{test:2} and run it, then go back to the second pane and rerun RETURN $test and it will still return 1.
So, it appears that the pane is storing its own instance of parameters. In the old Browser interface, parameters are global which seems more correct.
Was this an intentional change?