Red blood cells contain alot of potassium. In fact if a blood sample is improperly taken, too tight a tourniquet or improperly stored so that the cells burst,
the potassium reading is factitiously elevated for that test (falsely high).
The red cells dump the potassium into the serum where it reads inaccurately.

So the reverse also occurs during rapid cell formation. As the cells are stimulated to grow faster, they consume the potassium. If they are not doing that (no megaloblastic anemia is present), then the B12 goes elsewhere, and hence no fall in potassium occurs. We need to consume quite a bit from diet daily--- the new suggestions are 4.7 grams a day.

People can lose potassium thru the kidneys if they have renal tubular acidosis, take steroids, or have chronic diarrhea/vomiting. Diuretics given for blood pressure also deplete potassium (except for Dyrenium and Spironolactone).

I think that is why the literature has so few reported cases. The patient has to be in a medical crisis of sorts already, to react to B12 with hypokalemia. And I would expect injected doses to be more problematic with it.