The root of this problem can be that the Candidate has not clicked the Submit button in Workday after signing in Adobe Sign.
Per Workday task Check eSignature Signing Status: "To start the process, the user can submit the associated Inbox task."
Per Workday Development: The original signing completes the process only if the user submits the Inbox task after signing the document. The signers have to always submit the Inbox task in Workday after they complete the signing on Adobe Sign. Post signing, the iframe is closed and the user is redirected to the same task where they can click the submit button to complete the process.