This weird behavior occurs in VBA, or by sorting manually and is repeatable:

Enter a value in cell A1 and A2
Select Column A
Sort Column A (Data/Sort)

Now clear or enter space in cell A2 so only cell A1 is filled in
Select Column A
Sort Column A

The value in cell A1 is moved to cell A2

This only happens if cells A1 and A2 have values and are sorted first. If only A1 is filled in and sorted first, the move does not happen

If you are wondering why I'd want to do a sort when only one cell is filled, I came upon this using VBA where a column may have one or more cells filled in. My workaround was to check to see if there was more then one cell filled before doing the sort. But why is this happening and is there a real fix?