The victims of Alzheimer's (early or late stage) always think they are normal and everybody is lying to them.
This is my theory and I'm sticking to it: When a person has their outer veneer breached by alcohol, trauma, drugs or illness, the inner self is revealed; some people are absolute angels, while others are nothing but a vexation to all around them.
Google AI's telling me, at least in the early stages, those experiencing Alzheimer's may be aware of their decline. I'd agree, self-awareness is a tricky/elusive thing. Gradual changes, ones you're in the midst of, are hard to see.