Zhongli is known for being forgetful. Of course it might just be a quirky fact about him, but I think it could also be due to his age. He is the oldest of the Archons and having moved to a more human and mortal-like body might come with a few issues. Memory issues could be one of those said issues. He remembers Liyue’s history perfectly, things from the past, but has trouble with trivial and every day things of his present, similar to memory issues that can happen due to aging.
Also, I wouldn’t be surprised if he had something alone the lines of Adhd or autism, with his behaviour when it comes to things he is interested in (similar to hyperfixations) where he knows everything down to the bone about a certain topic and could go on about it for hours