Yeah, the title pretty much says it. I was standing (in multiple occurences) still next to the "output" of a spawner trap and hitting zombies with a sword. When I switched slots away from the sword and back, in some situations the sweep attack stopped working until I moved a bit.
Possible cause: The game determines the moved way/the speed only while the sword is hold and can't handle 0.
Possible fix: If 0, trigger sweep attack.