Excel won't delete an item "-8000"

[Pgroot]

Excel 2019 won't delete an item "-8000". If I double-click I can backspace to erase but -8000 returns in that item when I tab to next column.  It is in Compatibility mode.

[#2]

I should not need to double-click to select a negative number, and if I use "delete" it does nothing.

I was finally able to get it to backspace over the -800 and leave just 0, and then I used Enter.

That succeeded in leaving a 0 in the cell, but it also jumped to a different spreadsheet.  I don't have any idea why, but Enter has done that frequently.

[#3]

I should mention that I used Excel 2003 for years and never had a problem selecting or deleting a negative number, nor did I have problems with Enter jumping to another spreadsheet.

I am able to delete other numbers, but when I put another -8000 in a cell I saw the problem again.