So I was playing skirmish with the AI and used perimeter overwatch, however during the start of it my artillery sighters died and the effect cancelled. However even after 10 minutes any artillery which has the join bombardment button activated or any new artillery sighters recruited have the 'Perimeter overwatch must not be active' restriction on them. So I couldn't use it for the rest of the battle and had to destroy the 25 pounder artillery that was stuck in join mode.