The following discussion on Apple Podcasts may interest you.
podcasts.apple.com/gb/podca...
It’s about oral health and its affect on the body with an emphasis on root canals and dead teeth. They also discuss how gums are an entry point for harmful microbes, connecting to the gut. Intestinal permeability and low acidity are mentioned.
Not specific to b12 deficiency but no doubt related in the context of mouth/gut health generally.