Reports :: Report Won't Sort By Date When Query Contains Date Formatting

A few months ago I created a report that displays the results of a long union query comprising a dozen or so individual queries, each containing an expression that yields a date (or sometimes date and time). I set the report to group by query and then sort by the date expression. Now for some reason that I can't fathom the report has always only ever offered me the option to sort the date "A to Z", I infer it thinks the date is text, but this misunderstanding has never actually stopped it sorting by date perfectly well. It worked. No problems.

However I have recently added formatting to some of the queries so that they just display date, not date and time e.g. Format([dateandtime],"dd/mm/yyyy"), and now the sort by date in the report no longer works. None of the sorting or grouping options have changed, but it now sorts just by the "dd" component of the date - so it thinks 21st June is later than 20th July. why?