To reproduce:
- Add RadCalendar to a blank form and set its AllowMultipleSelect property to true.
- Select a date and deselect it right after that. You will notice that the background is not changed.
- The cell is actually deselected but it appears as selected since it is the focused cell and the styles are equal.

Workaround: Change the style for the focused state, which will allow the user to distinguish between focused and selected cells.