When you migrate to new servers, you have to be aware of the following limitation, which is documented in 12.0.2/14.0 but also affects older servers.
To ensure you can recover all user.IDs make sure the server document is still present and the server is still in the ID vault configuration. See the following warning in help and Kbase document.
This is a recent update in documentation and I just sent it to a customer during a server upgrade/move workshop.
This is a companion discussion topic for the original entry at https://blog.nashcom.de/nashcomblog.nsf/dx/important-domino-id-vault-dont-remove-old-servers-if-still-referenced-in-user-documents.htm