Depending on the version of Timeslips, that can be caused by a myriad of reasons.  You are not in limbo at all.

Bottom line?  It's not an accurate reflection of the client's status.  The status is not to be trusted as accurate.  The client's Accounts Receivable area will be where you look for a balance and status of paid/unpaid invoices.  The Bills dropdown (Bill Stage) is the area to look for a Proof / Revision / Undoable (a/k/a Approved) bill.  Additionally, the Billing Assistant will give insight as to all of this on one screen.

There could be some damage to the client AR balance or it could be a payment is not applied in the payments list for this client, or it could be nothing. If Aged AR report, Billing Assistant and the balance on the AR screen of the customer all show balance of zero, then I would not worry about it. If Aged AR shows balance of zero but a positive and negative number, then you have an unapplied payment. That can easily be fixed from the transaction list. Find the transaction that is not applied and apply it.