This almost certainly happens because of the import settings used when you opened the CSV file in the first place. Open it newly and find the "date" column, then play with the "column type" settings. (Text, dates, etc.) One of the "date" settings should be what you want. Also the check box "detect special numbers" may be applicable—I don't know what other data you are dealing with.
When dates are stored as dates, not as text, they will definitely sort correctly.
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