Invader lairs used to last until after the invader was gone from the kingdom.  Now, it seems I cannot even farm an entire lair before it is removed.  I can even [try to] send troops while others are still returning from a yield, but "the resource is no longer at the coordinates" or something similar pops up.  A fresh level 6 invader kill may leave a lair with 2.2m iron, but after making only 2 trips yielding 150k each, the third attempt to yield/farm the lair results in the "coordinates" message.  As an additional observation, the invader may still be active for 6 or more hours.  Today it happened on Barbarians, but this has happened to me on Serpent lairs, Northern Vulture, Cave Lion, and Canis too.