Two incidents in Ch12 make me wonder whether Anne is becoming open to the possibility of a second attachment. Here is her second encounter with William Elliot:

This second meeting, short as it was, also proved again, by the gentleman's looks, that he thought hers very lovely, and by the readiness and propriety of his apologies, that he was a man of exceedingly good manners. He seemed about thirty, and though not handsome, had an agreeable person. Anne felt that she should like to know who he was.

Also, when Benwick is attending her to the chaise back to Uppercross:

...she felt an increasing degree of good-will towards him, and a pleasure even in thinking that it might, perhaps, be the occasion of continuing their acquaintance.

Is she detaching from Frederick Wentworth? She has become “hardened” to being in his company and perhaps accepting/resigned to the likelihood that he will marry Louisa. There may also be a degree of detachment in the following passage, which has a tone of dry irony almost worthy of an Elinor Dashwood:

Anne wondered whether it ever occurred to him now, to question the justness of his own previous opinion as to the universal felicity and advantage of firmness of character; and whether it might not strike him that, like all other qualities of the mind, it should have its proportions and limits. She thought it could scarcely escape him to feel that a persuadable temper might sometimes be as much in favour of happiness as a very resolute character.

And yet Wentworth's attention and good opinion still affect her intensely:

...but if Anne will stay, no one so proper, so capable as Anne."
   She paused a moment to recover from the emotion of hearing herself so spoken of. The other two warmly agreed to what he said, and she then appeared.
   "You will stay, I am sure; you will stay and nurse her," cried he, turning to her and speaking with a glow, and yet a gentleness, which seemed almost restoring the past. She coloured deeply, and he recollected himself and moved away.
And:

But the remembrance of the appeal remained a pleasure to her -- as a proof of friendship, and of deference for her judgment, a great pleasure; and when it became a sort of parting proof, its value did not lessen.