Thanks for the code.
I also tried this:
if (this.pos.x == 49) {this.move(LEFT);}
if (this.pos.x == 0 ) {this.move(RIGHT);}
if (this.pos.y == 49) {this.move(TOP);}
if (this.pos.y == 0 ) {this.move(BOTTOM);}
And it also seems to work.
My problem was made worse when I skipped all creeps with fatigue > 0. But creeps with 0 < fatigue < 1 seem to be able to move so now I only skip creeps with fatigue >=1.