Based on what I understand, formal nouns like はず take up a subordinate clause, therefore taking が to mark the subject, but when I see examples such as
あの人は若いはず
or
あの本は高かったはずだ
It makes it look like my understanding is wrong.
When I tried to contemplate why, I just imagine that あの人 and あの本 aren't a part of the subordinate clause, but it is more of saying "that person/book is ～ はずだ".
If that may be the case can this also be applied to other formal nouns such as わけ?
Technically, 高い and 高かった are the short subordinate clauses that modify はず. Japanese relative clauses can be very short. As you said, the word marked with は is working as a topic of the entire sentence.
あの人が若いはず lacks a topic, and this doesn't even look like a valid sentence to me. It's just a fragment roughly meaning "probable-ness of that person being young". Still, you can make the subordinate clause have a different subject using が:
彼はあの人が若いはずがないと思っている。
He thinks that person should not be young.
