Per NetSight Help Documents:
Prompt (default mode) - If a client encounters a new certificate that is does not trust, the user is prompted to either accept or reject the new certificate. If the server certificate is replaced and the user expects to see the new certificate, then they can accept the certificate if it is correct. If the server certificate has not been replaced and the client has inadvertently connected to a server that is not trusted, then the user can reject the certificate.
This means that when the certificate is trusted it will prompt the user because the certificate has changed.