I upvoted a review audit that I passed, but after I was told that I passed, the upvote I cast disappeared. Shouldn't upvotes on passed review audits like this remain?
There are multiple problems with preserving actions that were conducted on a review audit which make it a no-go for keeping upvotes that were cast there. You are welcome to return to the post and cast an upvote if you think it deserves one, but we cannot keep the one you cast in the audit (it is never even recorded in the first place).
Audits do not display the post in its real state. They modify the vote counts, the user who posted, the date it was posted, and other details to make it match the queue where the audit is being displayed. This can cause confusion and result in duplicate comments and other actions that may not have been made if it had been viewed in its original form. Some posts selected are even deleted and cannot accept these actions in the first place.
Audits are used multiple times. Typically, when a post is selected as audit-worthy, it is repeatedly used as an audit for around a month. We would not want a post's state to be altered or flooded with excess actions simply because it was being used to test reviewers. For example, the specific post you mentioned has been used as an audit five times now - and will probably continue being used as one in the near future.
Not all users want their review audit actions to be recorded. Some people can easily detect when a review is actually an audit and simply select the option that will make it pass. They would not necessarily complete that action (such as upvoting the post) if they encountered the post in the wild while browsing the site but know that is the correct answer needed to pass. They would then be tasked with going and manually undoing whatever action if we recorded it as part of the test.