Perhaps check if another automatic hold has been applied after you released the prior hold?
On the credit check rule, are you using Override Manual Release. If yes, are you within the number of Days to Honor Manual Release?
If no, per Doc ID 1957309.1, it will "rerun" credit check and apply a hold if needed.
You can use the newly released EBS Order Management Credit Check Analyzer (Doc ID 2493364.1) to review the order and why it is still on hold.
Hope that helps,