What's the mechanism here? Because it's not like there is B12 supplements in the water.
if you only drink ro water it can creep up on you, but takes some time
refs:
https://www.epa.gov/sites/default/files/2016-09/documents/co...
https://www.cdc.gov/nutrition/php/data-research/fast-facts-w...
Rather if they did there'd be probably quite a lot of concern.
Not sure where that old wive's tale came from but even my parents had similar concerns against filtered water ages ago.