Deleted partially completed works orders from the plan and did not save changes reopened plan and the works order vanished

Accidently deleted a partially completed works order from the plan, but didn't save the changes. When reopened the plan, the works order disappeared.

When going to the Actuals tab, changing status to closed and clicking Apply from Edit Tasks (left-hand pane). The plan automatically saves. When you close the plan, you are prompted to save changes. Select No, so that when you reopen the plan, the deleted works order reappears.