Another edge case most likely.

Add a new category to a post via the web admin.
In the app, refresh the post list.
Edit the post. Notice the category you added via the admin is included in the posts categories.
Tap the + button to add a new category.
Without refreshing the category list, tap the back button.
Since the new category did not exist in the apps copy of the category list it was removed from the post.

If you let the category list refresh before returning to the post editor the category will be retained.