The issue you experienced occurred because the deprecated option "Show partially booked days with the same background as the legend item" was removed in a update 11.3 of the Booking Calendar. This option was only applicable to legacy calendar skins.

To resolve the display issue with these dates, please wrap your Booking Calendar shortcode in an outer div element with the specified CSS class on the page containing the shortcode:

<div class="wpbc_timeslot_day_bg_as_available">
[booking resource_id=1]
</div>

Of course instead of [booking resource_id=1] have to be your shortcode.

Alternatively, you can add the CSS class wpbc_timeslot_day_bg_as_available to any existing container element on your page. Once added, please save your changes and test the page.

Another option is to select one of the available new calendar skins instead of the legacy skin currently in use.

Additionally, please consider updating your Pro version of the Booking Calendar. You can view the latest release details at https://wpbookingcalendar.com/wn/ and request the update at https://wpbookingcalendar.com/request-update/.