I had a look at the context in which the words 'reserve' and reserved' appear in the other novels. Here's what I found:.

JA uses the terms to describe people who keep their thoughts and feelings to themselves:-

But, Lizzy, you have been very sly, very reserved with me. How little did you tell me... (P&P ch.59)

Edmund .... wanted to know Fanny’s feelings. She had been used to consult him in every difficulty, ........ If she did not need counsel, she must need the comfort of communication. Fanny estranged from him, silent and reserved, was an unnatural state of things; (MP ch.35)

Though shyness may be the cause of a character being reserved, shyness and reserve are clearly different:-

Anne Elliott meeting Capt Benwick:
He was shy, and disposed to abstraction;
Then Anne gets him talking about poetry, and he opens up...
For, though shy, he did not seem reserved: it had rather the appearance of feelings glad to burst their usual restraints; (PER ch.20)

Many of JA's characters do not warm to people who are reserved, and prefer people who are more outgoing. For example, Darcy is 'reserved', in contrast to Col. Fitzwillam who is chatty and easy to converse with.

Mr. John Knightley ...... with reserved manners which prevented his being generally pleasing. (Emma ch.11)

Mr Knightley's perspective on Jane Fairfax:
She is reserved, more reserved, I think, than she used to be: And I love an open temper. (Emma ch.33)

Emma's perspective on Jane Fairfax:
She was, besides, which was the worst of all, so cold, so cautious! There was no getting at her real opinion. Wrapt up in a cloak of politeness, she seemed determined to hazard nothing. She was disgustingly, was suspiciously reserved.
If any thing could be more, where all was most, she was more reserved on the subject of Weymouth and the Dixons than any thing. She seemed bent on giving no real insight into Mr. Dixon's character, or her own value for his company, or opinion of the suitableness of the match. It was all general approbation and smoothness; nothing delineated or distinguished (Emma ch.20)

A character who is reserved may be perceived by others as being proud and above their company:-

Georgiana's reception of them was very civil, but attended with all that embarrassment which, though proceeding from shyness and the fear of doing wrong, would easily give to those who felt themselves inferior the belief of her being proud and reserved. (P&P ch.45)

Emma's perspective on Frank Churchill:
He seemed to have all the life and spirit, cheerful feelings, and social inclinations of his father, and nothing of the pride or reserve of Enscombe. (Emma ch.20)

Marianne does not suspect Edward of being proud,nor does she think that his shyness is the cause of his reserve. Her disapprobation of his reserved character stems from her conviction that a person of true sensibility ought to express all their thoughts and feelings, all the time. She is wrong of course, and has yet to grasp the idea that someone could have deep feelings but not choose to broadcast them indiscriminately.