I copy a cell value from one worksheet (it is a sum formula). I paste the value into a different sheet in the same workbook. If I arrow out of the cell or click in a different cell, all is well, but if I hit return, the next cell down is selected and the value in the target cell goes to zero. If I return to the target cell and press return, the value stays.

I've never seen this behavior before.

Excel 365.