Well, after some major league messing around,I managed to questport far away with the demon in tow, and then questported back next to the fortess for fairness. (It seems to questport out of non-travel zones, you actually have to pin a quest...) After questporting it now follows me around like a normal companion/bound demon. I have, however discovered something... interesting.
This demon
is two historical figures.No, there aren't two of it... but this one, single demon is in fact two identical historical figures. I noticed two of it listed as my companions, and upon checking legends mode (the data there was exported before I starting dfhacking this guy) it only shows up once in the list of historical figures, but it shows up as being twice related to the dark fortress (the first two historical figures, indeed) yet, both links divert to the same demon, not a copy, as "both" created the slab and stored it in the vault, but the vault only dealt with one.
So, apparently it double-lived in that dark fortress and it's currently my companion twice. Upon digging some more, it seems every Dark Fortress I'd entered to any extent (including just loading the site and not reaching any towers, in the case of this demon, at the time of the legends data export) has had not only its master relate twice to it, but one histfig for every "building" constructed within has been associated twice. I'd put it down to the new Legends Viewer being broken with its "related historical figures" display, if not for the fact the game agrees and the demon became my companion twice. I'll probably experiment on my new-version world, once I'm able, but for this, this is... yeah. Oh, and Legends viewer still sees this even on an old export of the world data, from it's never even been loaded into the new version - and still, the weirdness seems to affect each dark fortress that I've visited SINCE then.
What even.