In short, if the foster care case is closed and an adoption is disrupted, never use the "Reopen Case"->"Undo Foster Care". This makes it look like they were never discharged out of foster care - which they were and it needs to be recorded as such to keep your days care and statistics accurate.
Instead: Discharge the "Adoption Case" with an outcome of "Adoption Disrupted".
After the case is fully discharged, look under "Reopen Case" and a "Readmit" button will appear. You must use that option and readmit them as a new, separate (but linked) foster care case. This is the *only* option to keep your statistics and days care correct.