Turns out I was running it with gravity off. With the default vertical gravity, the BUBW falls much more slowly than you would expect, because when anything conducts electricity in TPT, it is changed to the element SPRK with ctype set by its previous type. And SPRK is effectively a solid element, regardless of its ctype, so elements currently being sparked are immobilized.
Why what, specifically, is happening? The BUBW gets sparked because, like normal WATR, it is conductive. It creates CO2 because that's what BUBW does. Particles drift out from the edges because the release of CO2 generates slight pressure. But they escape very slowly, because nothing can move while currently sparked, so they can only drift in between the SPRK waves.
Turns out I was running it with gravity off. With the default vertical gravity, the BUBW falls much more slowly than you would expect, because when anything conducts electricity in TPT, it is changed to the element SPRK with ctype set by its previous type. And SPRK is effectively a solid element, regardless of its ctype, so elements currently being sparked are immobilized.