How do I trigger an automation on a sheet that is not open?

The problem: The "on change" automation does not trigger despite the flag changing - this is happening on a sheet that is not running.

The situation: I have 2 sheets.

Sheet 1, The Active List, has a list of active employees.

Sheet 2, The Remove List, is populated via data uploader (replace). It contains employees who leave the company.

The Mechanics:

When data uploader runs, Sheet 2 is populated with terminated employees. Over on the Active list, I have an Index/match formula in a checkbox field that looks to see if the Active employee appears on Sheet 2. I'm finding that after I upload, the flag will checkbox will have changed to "checked" but the automation associated with it did not fire.

The automation:

If flag Changes to Checked, then record the date and move the employee.

The flag is changing but the automation does not trigger.

Why does my automation not trigger on Sheet 1 when the conditions are met due to changes based on a separate sheet? How do I fix this?