In order to save the drop down list in validity rules to an Excel file format you need to use a range of cells for the list of options as this is the only method for a drop down list that is available in Excel.
I suspect that Calc is seeing empty cells as unused and therefore does not save that part of the worksheet. I was able to confirm the problem in Sheet2 of the attached example. Untitled 22.xlsx
It seems to be possible to work around the problem by either
1). having a none empty cell at the end of your list, I entered “.” in Cell A21 on Sheet2_2, (" " might be OK).
2). set the background color of the cell at the end of your list, I set Cell A21 background to white on Sheet2_3
Unfortunately this workaround only seems to work if the document is saved from Calc. If you open the document using Excel it is OK but after save again from Excel then the blank cells will still be missing the drop down selection. If I think of a way around this I’ll post back again.
Note if you might have empty columns you could set the cell of every column after your last data row to avoid losing any empty columns of drop down lists.