I'm going to speculate -- Buckingham Palace announces that King Charles has "a form of cancer," but doesn't specify which, other than that it was not cancer of the prostate, which was what he went in for.
I'm guessing low-grade leukemia. It could be found incidentally on a routine blood test and might require treatment that would prevent him from meeting people in person for a while.
When people hear the diagnosis "leukemia," they think illnesses that kill quickly, but that's often not the case in people his age. Some types of chronic leukemia are prone to escalate later, but others are the sort you die with, not of. So it would explain releasing the diagnosis of cancer but not specifying the type.
@EileenKCarpenter A former coworker of mine, who for a number of years was a firefighter, developed leukemia ( a major danger in that job https://www.firefightingincanada.com/cancer-in-firefighters/) damn near died from it and the treatment (killing off his immune system then rebuilding it with a bone marrow transplant.) He was young and in otherwise good health. Charles is neither - so it may be fatal either way - could be why Harry rushed back to England.
@NorthernInvader
There is a wide variety of types of malignancies that show up on blood tests. Some are very "indolent," not causing serious problems for years. So it could be it's something bad, but it could also be something that sounds worse than it is. The phrase a "type of cancer" is often used for leukemias, as cancer makes people think of solid tumors.