Update 4.1 | v0.8.5 (b598).

That demolition task is only accessible by amphibious trucks... but I don't have any, so it's causing Vehicles Status to start indicating progressively busier simply because there's a job on the queue- even though there's 60 trucks standing idle, and they absolutely can fit between the water and the rails; they did to get the rest of that job.

That particular job is actually a piece of train tracks right an the edge of the world, created by building right off the side of the terrain for as far as it'll let me, but that's not important for reproduction, just that it's only accessible by amphibious trucks. Which I did confirm, this does it too (Shattered Isles spawn location A, you can even see some of the idle trucks):

I've only triggered it with train tracks, but as the limit seems to be just 4 tiles away from where trucks can drive, something like a conveyor belt could probably do it too- if deconstructed in shorter segments, the way train tracks are.

Also potentially important is that I did have amphibious trucks on this save at some point, but demolished them once I was done with them. If there's a lasting flag from that, it's been set.

I'd expect the Vehicles Status to show "Stable", as it will the instant I Unity-deconstruct that, because there are so many idle trucks, and no jobs for any extant truck to do.

That said, the Vehicles Status display has always seemed highly unintuitive to me. It always seems to flipflop between "stable" and "extremely overworked"... without the steps between really meaning much, which makes it really hard to tell exactly what some change to the logistics system actually did. Is it possible for the status display to be decided by the number of unsatisfied jobs (of size possibly decided by the min/max/avg capacity of unassigned vehicles, which are exclusively pickups on my map), rather than how long it's been since a truck was unable to find a job to do? Is it also possible for it to separate them by type (land vs ocean), so 5 amphibious trucks building a 40-segment suspension bridge across half the map won't tie the display up on "Extremely Busy" for years (it did, when building the massive bridge on my current save, though I was using 20 amphibious trucks), and make me keep wondering if my on-land truck logistics are backed up or not?

Possibly related: I've since noticed (after making some amphibious trucks for another bridge...) that if you dump right out to the edge of the world like in the first image, that demolition job is actually inaccessible even to amphibious trucks, since they can't path outside of the buildable world the way ships can- which can create amusing scenarios where an amphibious truck has to use a land bridge to get between two ship docks that both have World Map access for ships, simply because one of them is in a bay whose mouth is dumped right up to the edge of the buildable map.