Thanks for the bug report, I was able to reproduce this problem and fix it so it will be fixed in the next update for event calendar pro.
In the meantime you can fix it immediately if you run this sql command to update the stored procedure:
ALTER PROCEDURE [dbo].[sts_CalendarEvents_UpdateSoldTicketCount]
Author: Joe Audette
Last Modified: 2014-03-28
SoldSeats = COALESCE((
FROM [dbo].[sts_EventTicketOrder] t
(t.EventGuid = @ItemGuid
(t.[RecurrenceGuid] = @RecurrenceGuid
AND t.RecurrenceGuid <>'00000000-0000-0000-0000-000000000000')
AND @TicketIncludesRecur = 1
AND t.OrderStatusGuid <> '00000000-0000-0000-0000-000000000000'
AND t.OrderStatusGuid <> 'de3b9331-b98f-493f-be5e-926ffe5003bc'
[ItemGuid] = @ItemGuid
OR ([RecurrenceGuid] = @RecurrenceGuid AND TicketIncludesRecur = 1)
It won't fix events already marked as sold out but it should prevent that from happening again.