Complex sentence: The additional tail surface though, was attached to the rear-observes sliding canopy which pilots claimed still caused stability problems when the canopy was in the open position, which was often because the aircraft's main role was spotting.
Split into simpler sentences: The additional tail surface was attached to the rear observer's sliding canopy and pilots claimed there were still stability problems when the canopy was open. The canopy was often open because the aircraft's main role was spotting.