It must be DND specific and only to international arrivals. The crewing issue can't be the reason as the crew are neither AMS or DND based and if they have crew to operate to/from EDI, then that same crew would be going to DND for what must only be about 15 minutes more flying time, there won't be any hours issues. If there was then it's taken 6 months to come to light as well!
I flew this route in June, literally a week or so after it launched. It's great, however on arrival back to DND it's clear the airport is not set up for international arrivals. Immigration control is an old gate room (DND had 2, now has 1 active gate room). There is one desk for all and that leads through a knocked out wall to the baggage reclaim area, the same one as used by domestic arrivals. There is a pop up customs notice and a small table for searching, which was unmanned when I arrived and then one small door and you're back into the check in hall, the same route as a domestic arrival.
The issue must be surrounding UKBF as I really cannot think what else would affect this and cause so much secrecy. I firstly thought Dundee city would have no UKBF staff based there, however there is a port in Broughty Ferry which must have customs staff, so one could be sent from there to meet this 1 flight a day. It's a little far for an EDI or ABZ based staff member to travel, but then again, that could be the issue.