This occurs when the recurring gift is split between more than one designation. Use the below steps to update both at the same time before saving:
  1. Navigate to Revenue > Transaction Search
  2. Open the Recurring gift record
  3. At top left click Edit recurring gift
  4. Change the Amount to the new Amount
  5. Click on the blue hyperlink for Designation
  6. Update the Amounts to be split between the designations
  7. Click Save, then Save once more