@ksaj @Luna @Wikisteff
Lactose is a complex sugar common to various milks. Infants produce lactase, which is the enzyme that breaks it up into simple sugars that are digestible, in their small intestines.
Many people, but a global minority, continue to produce lactase into adulthood. Typically, they can trace their ancestry to parts of the world where dairy consumption has been common for a long time, such as northern Europe.
For those who can't, they can either get the milk with the lactose already broken up or removed, or take lactase as a pill. Otherwise, they can't digest lactose but their gut bacteria can, leading to digestive upset.
Fine-filtering removes the lactose and some of the water, further concentrating the other ingredients, such as the protein. And it's one of those proteins that appears to be responsible for the canker sores.
This is, as far as I can tell, a food allergy, not a digestion problem.
https://pubmed.ncbi.nlm.nih.gov/28410620/