Servers (or any client system) doesn't just 'disappear' from WSUS without a reason. Either maintenance was run, removing computers that had not communicated within a certain time period (30 days if using SCW, configurable if you're using WAM with default at 30 days), or someone manually deleted it, or you have a SusClientId issue from improper cloning of systems.
Run the following client side script on every affected client ONCE and everything should go back to normal.
https://www.ajtek.ca/wsus/client-machines-not-reporting-to-wsus-properly/