One of the symptoms of dementia is that those with the condition experience some significant loss of muscle control, especially of the facial muscles.
Because dementia is in actuality a loss of brain function including motor function,it is not uncommon when the dementia progresses to a moderate stage, for muscles of the face to become inert.
The above is one big reason why people with moderate-to-severe Alzheimer's disease "look different". For example, compare Dopey Joe Biden's facial expressions today to those of even the recent past. His upper lip has become almost totally frozen. When he tries to smile, he either smiles crookedly or exerts such extreme effort to move his facial muscles that he appears to be leering or grimacing.
It is somewhat shameful that so many people around Biden are ignoring how debilitated and increasingly lost he appears, both emotionally and intellectually - all because they want to use him for political purposes and have nobody to replace him before the election.