The same happens when a user is checking the hotel calendar, impossible to see the availability from there. It seems all white and bookable also if is already booked.
Please click on any date first, then try clicking the block icon - the day cell should get the light red color, it should stay if you refresh the page. If you select a date and click unblock the color should fade, it’ll disappear after the page refresh (also the dates will be blocked or unblocked in the date picker).