There isn't any date checking going on in the driver. It simple copies the covers based on the options. Pre 0.9.21, if you had the "cover upload" option but not the "always upload" option, the covers were not sent when you first sent the book.
As PeterT said, each of the 2.x.x firmware versions have had a bug with the covers. As you are on 2.3.1, the wrong cover is used if you open a new book and let the device go to sleep before returning to the home screen. If you return to the home screen before the device goes to sleep, the correct cover will be used.
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