This situation can occasionally occur when a document is open and has unsaved changes in the client application at the moment a server update is being applied.
A server update triggers a restart, which interrupts the connection with the client. As a result, the client application may freeze or crash, and any unsaved changes will be lost.
The data loss affects only the specific document that was in progress — all other saved data in the system remains unaffected.
Recommendations:
always ensure all users have finished their work before applying an update
save and close all open documents beforehand
ideally, notify users in advance about the planned maintenance window