I agree it was either wired wrong or mis-labeled.. on my coach the slots labeled SPARE do not even have electrical contacts, just a plastic fuse holders.
Now, on some of the boards there are unlabeled slots, which do have connectors, but nothing connected to them.. Or fuses in them... These are for "Spare circuits" some times they are labeled aux 1, aux 2 and so on as well.... I have a bunch of them on the house board... though there are not as many there as there was when it was new
When I fill a "Spare" I label it properly.