Is it common for prepositions to be objects/to function as nouns?

This is not permitted except in exceptional circumstances.

That sort of language is not allowed except in less formal situations.

I thought that perhaps 'except in' was a fixed phrase and what followed was in fact the object, but I couldn't find 'except in' in any dictionary.

Here is one of the definitions of except (not 'expect') as a conjunction:

otherwise than; but (fol. by an adv., phrase, or clause): 'well fortified except here'.
I don't think the underlined parts are functioning as nouns. I suppose they're adverbial, showing under what circumstances the thing is allowed/permitted. There's nothing special about "in", so I wouldn't expect to find "except in" listed separately in dictionaries:

"Parking is allowed except on Tuesdays."

"I can't sleep except at night."
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But if expect is a preposition then the words that follow must be functioning as a noun (object of preposition)... So are you suggesting expect is not a preposition?
 Mister Micawber's reply was promoted to an answer.