Cells in column not displaying value of cell's formula, instead showing dupes

I have been working with the same Workbooks and Spreadsheets using the same installation of Windows and Office [365 - 16.0.12730.20270] on the same PC for many months.

Out of the blue, cell values in a column that are dictated by a properly structured formula (which I've used every day) are not displaying the value of the formula calculation in the cell. Except for the top cell in the column. The top cell is displaying correctly - but all the cells below it in the column are showing the same value as the top cell as if the top cell had been copied/paste-values all the way down.

When I run "evaluate formula", it does everything correctly, but when it gets to the last step of the evaluation, instead of ending with the correct value it suddenly seems as though it is being overridden somehow and posts the value of the cell above. I have tried all the usual stuff like checking for issues, rebooting, looking at how the cells are formatted, clearing and re-typing formulas manually, et al.