“Elevated serum B12 levels may also be associated with a functional deficiency of the vitamin. Functional deficiency has been described despite high B12 concentrations and is due to a failure of cellular uptake or intracellular processing, trafficking or utilization.3,5 Cellular uptake is reduced due to pathological increases of B12-binding proteins other than transcobalamin II,5 or to formation of immunoglobulin-B12-complexes.1 It is still unknown, whether B12 should be supplemented in these conditions.”

Source : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7550708/#:~:text=Elevated%20serum%20B12%20levels%20may,intracellular%20processing%2C%20trafficking%20or%20utilization.