[Edited to give a simple one-paragraph description]
Using 24.04rc2 (b84fce4) with Qt6 on Windows 11 with system clock using a North American time zone that observes Daylight Saving Time (and the bug also occurred in version 23.12.1)
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Whenever the current time of day falls precisely within the one-hour time interval immediately after Anki’s “next day starts at” changeover hour (by default 4:00 am to 5:00 am), then when clicking Stats to open the Statistics window for any deck, in the Calendar section the little square for March 10, 2024 (which was the Daylight Saving Time changeover date in North America, namely the second Sunday in March) displays as white and a mouseover over it shows 0 reviews. At any other time of day, that square is blue as expected and mouseover shows the actual (non-zero) number of reviews.
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The little squares for the Daylight Saving Time changeover dates of previous years (2023 and earlier) do not show this behavior either for the March switch to daylight time or the November switch back to standard time.
PS,
The color change from very dark blue squares to lighter blue squares between the two images is associated with switching between “last 12 months” and “all history” radio buttons.