I think the enemy NPC AI needs to have a different reaction when all players are out of sight in combat. Currently, if the enemy NPCs can't see any players, they essentially just skip their turns. This make stealth an overpowered ability in combat, where you move slightly to fire a ranged attack, move back to a position out of enemy sight, use hide as a bonus action, and then every enemy skips their turn. Rinse and repeat until all enemies are dead.
This allows for a single character in combat to kill a whole group of enemies, often without getting attacked a single time. Instead, if they can't see any players on the start of their turn, the AI should have enemy NPCs either move in the direction that they last saw a player, or seek cover themselves. This better reflects how someone would actually respond when the person they are fighting seeks cover, by either attempting to pursue them, or by getting out of attack range themselves.
provided no vision cone intersects with you or a party member. I have moved my entire pay behind 1 character who just stood there...and I had a fighter wearing plate armour 
