I faced following problem / feature in WordPress:
Let’s assume that default category for new posts is A. When I create new post with category B, WordPress seems to save the new post to database with category A and rigth after save it changes it desired category (B).
I need to read new post and it’s category rigth after the new post is published at the first time. Every time I publish post it’s category is default (A), regardless whatever I select. WordPress mechanism seems to be working like this.