I'm using this great plugin when I bump on this problem/bug.
When I add a new post on the category, in the reorder plugin UI I can see the post that I added and can still reorder it but on my wordpress site it seems that the current count of the posts is still persisting.
Last post count is 8
added 1 new post in the category that makes it 9
but on the wordpress site it is 8 the last post in an order is missing.
I tried to refresh it by clicking Yes and No on the UI but still not working.