I have a great deal of personal experience with dementia as both my father and father-in-law passed from it.
Some people are VERY good at “masking” their affliction as was my father. He managed to deceive the family well into in disease before we became aware of his problem. But, there are always moments when the mask fails and the disease becomes apparent. The further the disease progresses, the more often and obvious the disease becomes.
I've seen it too. The mother of one of my friends doesn't rembember her kids or her beloved dog, but was cogent enough to do a street interview by a tv crew on local politics. And pulled it off.
I once had an elderly neighbor who seemed normal. Admittedly out interaction was limited mostly to exchanges over the fence when we were both working I our yards. One day she called me and asked if my cable TV service was out. I checked and told her mine was, too. About five minutes later she called and asked the same thing. I told her it was. About five minutes later she called and asked again. If the service hadn’t been restored right after that she might have called again...and again. The next time her daughter came to visit her I mentioned the incident and she confirmed her mother was in the early stages of Alzheimer’s disease. Steps were being finalized for her longtime care.