Consider the special circumstances of a patched Controller.
The patch process requires logging into the NSX Advanced Load Balancer CLI as the admin user.
Being able to log in as an admin user implies the factory-default admin password has been changed, as explained in the Strong Default Admin Password article.
A Controller which has had its admin password changed has a configuration.
As mentioned, follower nodes are expected to have no configuration, and no changes to the NSX Advanced Load Balancer login credentials. If follower nodes do have credentials, a message of the following form is displayed if one attempts to cluster them:
Cluster configuration failed. Unable to add node 22.214.171.124: Login with default credentials failed, clean reboot the node and retry.
Therefore, before joining two patched Controller followers to a patched Controller leader, run reboot clean commands on each follower.