Thank you for your advice, it's highly appreciated. And already on the todo list.
Enemies losing interest and returning to their patrol was intended. It's fine for normal enemies, giving the player the option to stealth their way through the dungeon. The guards in the Soul stone rooms however should be more difficult and obviously need some adjustments to their settings. Field of view should be larger, follow distance longer, follow speed higher... Maybe even a door that locks behind you after you enter the room. It should be harder to escape them, if not impossible.