Why after changing the flag on a AP batch to ready to post, transaction posted from PO keeps updating same batch ?

We have configured our system to do all costing during posting. and with multiple departments using the PO module we'd like to set a AP batch as ready to post to avoid modification.

Now what we have notice that even after setting AP batch as ready to post, once a entry is posted from PO it doesnt create a new AP batch but instead modify the exisitng batch which is already set as ready to post.

If we change the costing procedure to " during Day end processing" we are able to create a new batch after each Day End.

any suggestions How can we fix this ?