Cannot assign mappers while other devices are running

"Atlas mappers are currently in a map"
Except they're not, not in this device anyway (there are mappers running maps in the other two devices). No maps are even in the map slots of this particular device (Map device 2). I was going to replace the mappers and was able to remove all of them from the device, now can't put any back in except for 1, oddly enough. The one on top of the list was assignable and is also removable again.
The reason being that other devices are running is purely my speculation, it could be that a chicken needs to run into a particular spot in Kingsmarch for all I know, would not be surprised in the slightest.
PS5 Pro

Edit: I think I might have found the cause and if so, it's not a bug. Among available workers, not just the idling ones are offered when assigning them. The ones that are currently assigned to a job are there as well, mixed in willy nilly (the current status of each worker is noted in the list but yeah, unless you know that it's implemented like this, you probably won't be looking). If those are currently mapping in another device, they naturally won't be available to switch. So just a UI issue - a pretty terrible one, but we're used to having those, and it's not a bug at least then, just a questionable feature.

Edit 2: nope, this one's my bad - there's a check to only show idling ones. Frustrated myself into this one it seems, lol.