I really don't see how the payee field could affect this in any way so I think we should look for something else.
Have you create an account or deleted an account recently ?
When several transactions occur on the same day you can reorder them by drag & drop by clicking on the 2nd item of the bottom toolbar. In order to do that you must have added the 'Manual ordering by drag & drop' sort criterion (after 'Date' or 'Value date') before.