My brother has several types of dementia, he is starting to not see a certain item, but can see everything else. Has anyone else encountered?

There is a subset of Alzheimer’s dementia that involves the posterior lobes of the brain (Benson’s syndrome) and sight is affected in this manner.

Strokes can also affect sight. That can be vascular dementia.

Do you know which dementias he has been diagnosed with?

Visual agnosia I've seen others mention it on this forum plus we had a man in our church who suffered from it, he was only 65-ish. https://my.clevelandclinic.org/health/diseases/23421-visual-agnosia

No, I haven't seen the "not seeing thing" if you mean he cannot see, say, a telephone. Now he may not recognize the WORD for it or connect language for it. There is a big difference. That would result in the sort of thing where you said "Where is the phone, Dad" when it is right in front of him. He may not connect the word and object. But if you said "can you lift this up" and point to it, he should be able to "see" it OK.

I don't understand your statement, "he is starting to not see a certain item......."??????? What does that mean?

Please elaborate so you can get some decent comments.