The option to Sort by Column A or B has disappeared

I must have done something here without realising. Ever since I can remember, I have been able to Sort selected rows by choosing the Column to sort by, usually Column A. But, for some reason, the option to sort by Column A or B has disappeared and been replaced by From and To (see screenshot).

It may be because I duplicated the first Header Column A, thus creating two Header columns A and B. So I selected them and went to the menu Table/Header Columns and set that to zero, thinking this might release the Sort capability, but it hasn't.

Can anyone show me how to get back the ability to Sort by Column A?