The behavior is variable according to the calendar source:

Connection Between Stays.net ListingsConnection to External Listings
The block will be removed as we were able to identify the source of the block and which Stays.net event to delete from your calendar that is related to the removed block.The block must be manually removed, as events from external channels do not come with identifiers and as a result, we are unable to find the block related to the deleted activity on your external portal.