In a post-function, you will see the current status of the issue - the issue object has been read at the start of the transition and the status won't change until the end of the transition where the issue gets stored in the database with its new values.
So yes, that's exactly what I'd expect to happen.
I did some more reading. Atlassian makes this statement:
Post functions carry out any additional processing required after a Jira workflow transition is executed, such as:
I am back to figuring out why I don't see the status updates in my post function.
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