No. It is a bug with the RadCalendar. The RadCalendar never believed that it could go disabled once the selection changes but before the mouse is released so it has some internal issues. It could even get worst if the selection is Multiple or Extended as it would continue to select dates when it get enabled even if meanwhile the mouse is released. We had that problem reported once but it is not a common issue. Our team is focused on other controls at the moment so I can not tell you if we are going to fix it.
As a workaround I could ask you to start the heavy lifting in a timer after few milliseconds time or add a "GO" button near the calendar which have to be clicked in order the data for the selected date to be generated. Also you could consider putting the calendar outside the RadBusyIndicator. For example if you have RadCalendar and a RadGrid near it that shows data for the selected date you could leave the RadCalendar clickable while the busy indicator contains the RadGrid and disable it while loading its data.
the Telerik team
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