Hello,
This udpate KB5035963 is a SSU which is the prerequsite to install further monthly updates, so amybe it is install together with other cumulative updates.
There is also another possibility that user manually downloaded it. Even the machine update is managed by WSUS, sometimes user still can manually install update, for example, click setting/ windows udpate/ check online for updates. This option will let the machine directly download update from Internet instead of the wsus server.
Best regards,
Molly
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