I know I used to know a word for this, but I'm not sure what it was.

Interestingly I can't find a german word for this and usually we germans have an unspeakable long word for everything. weird.

Are you looking for the word, "forgotten"?

I get this. I call it "deja durrr".

Getting old

Alzheimer's

Gone but not remembered.

There's probably some really specific word in German for this, but I can't think of a single word in English that would fully describe what you're talking about.

Love the concept, don't know the name. Is "evanescent" approaching (I know it's an adjective but maybe it helps)?

Presque Vu maybe?

That totally happened to me the other day when I was visiting the town I grew up in. It's only a little over an hour away by car, but I hadn't walked around the downtown in years, and it's so different that I couldn't even remember what buildings used to be when they were replaced sometimes.

In New Orleans you'll often hear "Ain't there no more."

As in, "hey wasn't there a great little cafe here before the storm?" "Yeah, but aint dere no mo"

saudade

edit: also, differently, mono no aware

Reminds me a bit of Jamais Vu

It's not a perfect match to what you describe but it's defined as "the phenomenon of experiencing a situation that one recognizes in some fashion, but that nonetheless seems novel and unfamiliar."

In your case, you recognize there was a building, and recognize it should be familiar, but it still feels unfamiliar to you nevertheless.

Hopefully this is in the right direction lmao

Dementia

Edit: I figured for sure that giving the wrong answer would lead to the right answer immediately showing up in the replies... Sadly, I was wrong

"Passed out of memory"

Alzheimer’s

Life after the monitor

This guy probably created one https://www.dictionaryofobscuresorrows.com/