From https://www.sciencedirect.com/topics/medicine-and-dentistry/ca-19-9-antigen#:~:text=Interference%20of%20heterophilic%20antibodies%20causing,of%20CA%2D19%2D9.

"Elevated CA-19-9 levels usually precede the radiographic appearance of recurrent disease, but confirmation of disease progression should be pursued with imaging studies and/or biopsy. CA-19-9 can be elevated in many types of gastrointestinal cancer, such as colorectal cancer, esophageal cancer, and hepatocellular carcinoma. Apart from cancer, elevated levels may also occur in pancreatitis, cirrhosis, and diseases of the bile ducts. It can be elevated in people with obstruction of the bile duct. In patients who lack the Lewis antigen (a blood type protein on red blood cells), which is about 10% of the Caucasian population, CA-19-9 is not expressed even in those with large tumors. This is due to deficiency of the fucosyltransferase enzyme that is needed to produce CA-19-9 as well as the Lewis antigen. The use of a combined index of serum CA-19-9 and CEA (CA-19-9+[CEA×40]) has also been proposed for screening of cholangiocarcinoma.

"Interference of heterophilic antibodies causing false positive CA-19-9 results has been documented, and usually treating the specimen with heterophilic antibody-blocking agents can eliminate such interference. Patients with acute or chronic pancreatitis may also have elevated levels of CA-19-9. In addition, pulmonary diseases may also elevate CA-19-9 levels. Liver cirrhosis, Crohn’s disease, and benign gastrointestinal diseases can also increase CA-19-9 levels."

I'm not a medical professional; I just googled for reasons why CA 19-9 could be elevated. There are a bunch. That's what makes CA 19-9 a tricky tool to use for definitive diagnosis. On the other hand, I've always heard that the CA results are more reliable to track disease/treatment progress once you have a firm diagnosis. That's why, if my CA 19-9 were starting to rise, I would be pestering my oncologist for more assessment to pin down why.