The absence of any one symptom or collection of symptoms cannot rule out B12 deficiency.
Most striking perhaps, because of multiple factors an undernourished person can even gain weight. Many think an undernourished person must be wasting away.
This is excerpted from a mainstream medical textbook, one of the few reliable sources of information:
Goldman: Cecil Textbook of Medicine, 22nd ed., W. B. Saunders Company,
Chapter 175, page 1054
None of these abnormalities are specific for cobalamin deficiency [B12 deficiency], and any of them may be present alone or in any combination and may vary greatly from patient to patient. None of the abnormalities are always seen in cobalamin deficiency, and the absence of any one or a combination of them does not exclude cobalamin deficiency. The neuropsychiatric abnormalities may occur early or late in the course of cobalamin deficiency and with or without any of the hematologic or other abnormalities . . . How the deficiency of a single substance, such as cobalamin, can produce a clinical picture with such wide variations in severity and dissociation of various hematologic [regarding blood] and neuropsychiatric abnormalities is unknown.
[bolding and brackets added by rose]
More to come...