Lactobacillus reuteri occurs naturally in some human breast milk. But:
In the 1960s, when the bacterium was discovered, L. reuteri occurred naturally in the bodies of 30-40 percent of the population. Today it is found in 10-20 percent.
And, from the same source (
"On average one of seven women had the bacterium in their breast milk. In Japan and Korea, however, women had higher concentrations of lactobacilli," says Sinkiewicz, who says that the prevalence of L. reuteri in breast milk is important, as it helps the infant's intestinal system to mature and its immune defense to develop. She also maintains that it affects the risk of developing allergies.
IN 2009, a study (which for some reason was performed on mice and not humans) found that Lactobacillus reuteri (more specifically, the CRL 1098 strain), "prevented the symptoms observed in female and weaned young animals fed with a nutritional B(12)-deficient diet". The conclusion of the study was that "Our data suggest that the pseudovitamin B(12) produced by L. reuteri CRL1098 is biologically active and effective in preventing the pathologies caused by the nutritional deficiency of B(12) both in pregnant mice and their offspring." Another article, posted a few months later, suggested that "evidence that pseudovitamin B12 is biologically active in mammals is still lacking", so in addition to the unfortunate fact that this study was performed on mice and not humans, there's no proof that the existence of bacteria found in some breast milk actually prevents low B12 absorption from causing the symptoms usually associated with a low B12 intake.

Further studies could probably both find out why the natural occurance of the Lactobacillus reuter bacteria is reduced since the 1960s, and if the B12 produced by Lactobacillus reuteri actually is biologically active in humans. It could be as simple as this: "Humans not exposed to any significantamounts of antibiotics or 'natural' bacteria/B12 killers could have a natural occurance of Lactobacillus reuteri in their digestive system", while others don't. Or: "Humans breastfed for more than a certain amount of months by a mother not deficienct in Lactobacillus reuteri have a natural, healthy level of this bacteria, meaning a reduced need for intake of external B12".