I have created a few custom npc heroes to recruit and made it so they had really good vision. 25, which kinda may be overkill, but hey, I like seeing whats going on around me. Anyway, if I leave these heroes sitting still in a town or just hanging out on a random part of the map the vision goes away after a few turns. One second I can see a good chunk of the map, the next its like my heroes go blind. The fog of war swallows up everything and I have to physically move the unit to get its vision back to the way it was. Any idea what causes this?