Even worse, deleted attachments are not deleted.
You can see it if you add an attachment via the API, store the ID, link it to an event then delete the attachment in the bunq-app. If you then link the stored ID to the same or another event you'll see that the attachment is available in the bunq app again.
This should not be possible if the attachment was really deleted. The bunq app only seems to remove the link between the attachment and the event. Same applies for the search index, attachment is unlinked but the search index is not updated.