• Dack and Quin are the only unique NPCs I can find in the game who do not actually have the UNIQUE flag set to TRUE.
  
• The "Scraphouse" town radius is astoundingly tiny and, even more bizarrely, DOES NOT include the actual warehouse floor
• everyone in the actual Scraphouse shop is, in Vanilla, OUTSIDE THE TOWN (see screenshot...)
  
• At the same time, the Black Desert City radius is freakin' huge and actually OVERLAPS the Scraphouse town circle
  
• The AI package set for the Scraphouse residents was the generic "Shop 24hr" package used by bunches of other stores, such as the Tech Hunter Waystation shops.
  
• That package contains a number of things we don't want, such as an AI Goal for the squad leader to "take intruders outside." Quin is the squad leader. We don't want him wandering off into the Deadlands with unconscious Iron Spiders or something...
  
• Being a generic package, its signal is set to check SAFE_TOWN, not HOME_TOWN
  
• The Scraphouse is correctly set to "Public Day," but the "Public Night" flag is set to FALSE, unlike every other 24hr building such as the BDC Robotics Shop and other places that never close.

All of these taken together throw red flags for me. They stand out: they're different than all other similar items.
The plain fact is, we do not have complete information on how all of these factors work under the hood, much less how they interact.
So while some or even many of these may not be a factor in this specific bug, there's just no way to tell short of inside information from LoFi.

The simplest and most rational next step is to eliminate all the weird &()$#@?!! and then look at how it behaves afterwards. Therefore:

• Dack and Quin are now flagged UNIQUE
  
• BDC radius was very slightly shrunk, to get its border off the Scraphouse structure and away from the Scraphouse town area.
  
• Scraphouse radius was increased 50%, to cover the entire warehouse floor.
  
• The Scraphouse town marker was moved to the CENTER of the warehouse, because c'mon now
  
• The "Shop 24hr" generic AI package for the Scraphouse squad was replaced with its own unique copy
  
• The Scraphouse AI package was trimmed of useless stuff like "man turrets," and most especially things like "have Quin carry people outside"
  
• It's now the Soldierbots' job to carry out intruders, should there be any
  
• Public flags set for both day and night, matching every other 24hr shop in the world

Quin's vanishing act has been a thing forever, and AFAIK he is the only NPC to do so... and he has done so for many players, repeatedly, requiring an import to bring him back.
Therefore it is reasonable to assume that something unique in the combination of settings and factors here is causing it, as Quin himself is a "normal" character with nothing that could explain his wandering off spontaneously.

By the same token, a simple "failure to load assets" doesn't really fit, since the Security Spiders and Soldierbots and Dack all wander around much further than Quin does, and yet they do not disappear.

And lastly, it is notable that once Quin vanishes (causing Dack to be nominated as squad leader) Dack becomes subject to the same bug, sometimes himself disappearing. So this is a further pointer to the squad's AI and its interactions with the town itself.