When WIP gets picked up in a bill, it becomes billed time. If that bill then gets reversed, the system reverses the billed time and reinstates the WIP. So what you are seeing is a contra entry. i.e.
£100.00 WIP, then gets picked up in a bill.
(£100.00) Billed Time.
£100.00 reinstated WIP.
This is how the system works by design, to show a clear audit trail as to what has happened with a particular item of time.
From a billing perspective, this can look convoluted if you were then to say pick up all 3 items in a bill to a client, To get around this, so not to include the contra entries in the actual bill to a client, Pick up the original WIP together with the reversed WIP and post this in an internal bill which would net to zero, for good housekeeping. This leave 1 item of WIP to then be picked up in your actual bill to your client.