I have noticed that frequently when I tell a companion to go check out a bed that they will tell me it's impossible. Like, for example, if they need to go up stairs (specifically the stairs from the wooden floor selection in a 2 by 2 tower layout). However, they have no problem when I ask them to go down the same stairs to check out the same bed (I have multiple floors of this, and tested on of the beds both ways).

So that may or may not be an error in the game's implementation.

Meanwhile, in another context (and an earlier release), I coated a floor with sleeping bags. And I got a spike in population followed by an immediate dive. I presumed that they gave up and left my settlement when they discovered they had pathing problems to reach their assigned beds.

So I am wondering if pathing issues might have something to do with observed settlement "decay" problems? (Probably not the only issue there, of course...)

pathing issues could also be caused by these recent mods that change the way settlement are constructed, this not ignoring the fact that the are some of these kind of bugs in the game as is. Are you using any such mods?