Symptoms

Cannot configure credentials on list item activity, warning is surfaced "Unable to validate the user name - The Method or operation is not implemented"
 

Diagnoses

Run as can only work with windows authentication and in this case the underlying broker (CSOM) is configured for Oauth. Oauth again is needed for K2 Service account to have the ability to perform tasks in SP on behalf of. In Workflow context all executing is done by the service account (Account owning the Oauth token) as this account has the Admin token for Oauth (the account that ran trusted the app ). This is by design. Workflow Service is an async service which doesn't have context to the connected user's security credentials. There is no impersonation that can be done to execute the underlying broker as the current user.
 

Resolution

Currently by Design. Enhancement logged to make the message clear and/or remove the option to configure runas on Oauth based services.