Can methylcobalamine b12 cause anaphylaxis?

Yes: It may be rare but it does cause it and that is why the injectable form is usually avoided as the oral form can cause allergies first as in allergies to certain food and can easily be avoided as a consequence.

Unlikely: I have never heard of methylcobalamin causing anaphylaxis and given that it is a chemical that is naturally found in every cell of your body it is probably impossible to have such a reaction. However, a supplement containing methylcobalamin may contain fillers and binders that some could be allergic to, so one could theoretically have anaphylaxis to a B12 supplement caused by the other ingredients.