Why is ままだったはずだ rather than ままのはずだった used in this context?

Question

(十香)「何を言っているのだ？確かにあやつの行動は目に余るが、前と全く同じというわけではないぞ」

(士道)「そうか？むしろ前よりヘビーになってる気がするんだが……」

(十香)「だが前の折紙なら、私が無理矢理引きはがすまでシドーにくっついたままだったはずだ。それに、今立ち去るときの顔は……」

Hi. Context: 士道 and 十香 were talking about 折紙. 折紙 embraced 士道 and let go of him voluntarily. 十香 was surprised about 折紙’s behavior. Due to some reason, 折紙’s personality had changed a lot. 十香 thought 折紙 was not what she used to be because if the present 折紙 were the past 折紙, she would have sticked to /embraced 士道 persistently until 十香 forced to separate them.

Given the context, shouldn’t we use ままのはずだった rather than ままだったはずだ? The difference between the two expression is as follows:

「XXだったはずだ」 -- "it must have been XX" "I am sure it was XX" (as far as I can remember, XX must have happened or been the case, though what actually happened is contrary to my expectation)

「XXのはずだった」 -- "it should have been XX" "it was supposed to be XX" (but XX didn’t happen or wasn’t the case)

https://japanese.stackexchange.com/questions/36220/confusing-use-of-plain-copula-だ-should-be-だった/36261#36261

Do I get it wrong or this sentence is an exception? Thank you.

Please let me know if the context is not enough.

Answer 1

Great question. Let's start with the base line.

• 〜のはず: supposed to be 〜, probably 〜. The mental picture is that the speaker is referring to something they have no direct knowledge of, and they are pretty certain but with some doubt. The emphasis is on the presence of a doubt.

• 〜だった is a past tense modifier. It just indicates that the stuff happened in the past, not now.

In くっついたままだったはず, はず decorates だった, meaning 十香 is pretty certain now of something that happened in the past. The context tells us not only 十香 currently has some doubt that it didn't happen, but 十香 actually already knows that it didn't happen. See, this phrase can cover a wide range of conviction levels, and it could be used both for "it must have been" and "it should have been", or even "IIRC, it was".

In contrast, in くっついたままのはずだった , だった decorates はず. The point in time when a conviction and a surprise happend was in the past. So the meaning changes to "折紙 was supposed to stick to 士道 [but we know now that it failed/it didn't]"