@sbt19 I wasn't informed specifically about the association of B12 deficiency with the hemicolectomy, but my PCP checks B12 levels annually as part of the lab testing. And following my surgery I was seeing an oncologist and they check lab results for every visit. I was also anemic ( mostly iron deficient, I think) and they checked B12 and folate levels as well. I received a couple of B12 shots there after the surgery, but my levels increased and stabilized and that's when I was instructed to take the B12 sublingual pills.

From what I'm reading, B12 is absorbed mainly in the terminal ilium, the last part of the small intestine. I've never been able to figure out if I still have my terminal ileum, it wasn't mentioned in the operative report, the last portion of the small intestine was attached directly to the remaining transverse colon after the removal of my cecum, ascending colon and portions of the transverse colon containing the tumor. Yet it looks as though the pathology report with the gross description of the tissue mentions the terminal ileum as present in that issue. So I don't know.

As you can see from the articles below, B12 absorption is a complex process and there are any number of issues that can arise that may compromise this absorption, this can occur whether or not the ileum is present. Often B12 deficiency is attributed, especially in older individuals, to not enough gastric acid to properly break down the foods, and may be seen in people who take a lot of antacids to control GERD and other gastric issues.
https://perniciousanemia.org/b12/absorption/
https://www.ebmconsult.com/articles/vitamin-b12-absorption-mechanism-intestine-intrinsic-factor