Why is text typed into other cells appearing in a cell with no formulas in a workbook with no macros?

I have a workbook with a few sheets of data, one PowerQuery, a few pivot tables.

On one of the sheets, when I type into some cells at the top of the workbook, whatever I type is repeated in cell B3. It appears large at first, then the text shown in B3 compresses as I type longer sentences.

There are no formulas in B3. There are no macros in the workbook. There is no code acting on the workbook in any way.

Has anyone seen this kind of thing before? Do you have any idea what might be causing this?

Please keep an eye on the formula bar. I type something in cell B3 then delete it. Then I type in other cells but the typed data is repeated in or over cell B3

EDIT: I’ve since learned that whatever I type in ANY cell in any sheet in the workbook is repeated in this way in/over cell B3 on every sheet. This includes newly created sheets.