Mine go through phases during the year as the water temp changes. This time of year they work near perfect, but as the water gets colder they can start to have similar issues.
When they won't turn off with the pole extended, first I lower the flow rate a little bit to reduce back pressure in the hose. If that doesn't work then I simply fold the hose in my fingers at the base of the pole to shut of the water when working at height and only use the Univalve working at ground level.
However, mine are from the original batches with the stronger springs so this might not be helpful. Might be worth a try though