Biters appear to despawn after some time of inactivity. This is probably fine in vanilla since the base will just spawn more of them, but when only a limited number are spawning it's an issue.
Possible fix: when enemies are spawned, assign them to a unit group and poll them to keep them patrolling their point-of-interest. This should also help them return to spawn when the player runs away.
Biters appear to despawn after some time of inactivity. This is probably fine in vanilla since the base will just spawn more of them, but when only a limited number are spawning it's an issue.
Possible fix: when enemies are spawned, assign them to a unit group and poll them to keep them patrolling their point-of-interest. This should also help them return to spawn when the player runs away.