IM hydroxocobalamin (B12) treatment

It is as you say, the B12 is there but the cells can't use it. When this happens, the levels of methylmalonic acid rises, as it is metabolized in a process dependent on B12.

 

There might be one nevertherless.

Obviously actions in the body take place in a range of degrees.
These actions will be boosted or slowed down by molecules to be released, or even atoms.
Furthermore, there are different influences on an action (directly and indirectly),
it´s a web (which must stay stable enough).

If now a certain action has been slowed down,
and is itself in a single cell nevertheless within a normal range,
though an aggregation of this kind of action is too small (and this would be not normal)
one might want to boost cells of this action, although everything behaves as normal,
and there is no deficiency.

In this logic it would be important not to counteract the wanted influence
(which easily will happen, as the body is a web of different influences).
Here it would be crucial to do the molecule or atom without other stuff to the body.
It could even be important to do it in small dose to the body,
otherwise other actions might start (or maybe the reception will get close for such a too high influence).