Overall, I have found they behave fairly "normally" (as normal as any pixelated false 3D algorithm-driven representation of a refugee in a retro-futuristic post-apocalyptic wasteland can be expected to act!).
I just finished redoing Oberland Station today. I've got this gravity defying two story cube that completely wraps around the upper floor of the railroad tower. The floor is about level with the eaves of the roof, two staircases to get up into it. I'd say it is somewhere between 9 and 12 squares (the larger ones) on each side. One 'row' of ceilings around the outer edge is one story and that is where their 'apartments' are located. the central roof is two stories so the whole inside is quite open and airy and I used those wall pieces that are just a lattice work with lots of holes for the whole upper wall, so it is as if it has a kind of 19th century factory with lots of big windows around the ceiling. Got it wired up my benches set up and everyone assigned (all top tier shops, one of each, and maybe one or two doing farming, only 11 settlers at this point). In the course of tearing down and rearranging everything, they had all gone to bed and by the next morning I had it all set up. Everyone got up, sandboxed their morning coffee and donut, eventually went to their shops, etc. I guess the farmers must be out there farming not sure. A couple of my shop keepers are unresponsive: I click E on them and they just repeat one of their canned lines and no option to barter or trade. But I suspect by stopping and restarting, maybe leave the place and come back that may fix. I did a lot of assignment shuffling so they might still be a bit confused. I think these two ladies that are responseive might be the original two that are there when you arrive so that might be it as well; maybe some of the "originals" don't function well in shop keeper roles?
So as you can see, working pretty much normal except some fairly minor glitches. A couple things I've noticed: pathfinding is pretty bad once you start building complex structures with stair cases and stuff. I originally had the two stair cases off set and their was a smaller "lower level" (basically a second story that is right above the workbench). I have the bell in the middle of the third story (the big open area with high ceilings). With the stairs offset (you had to go up one turn, go around a corner and then turn again to go up the 2nd staircase) a lot of them would make it into the second level and stop. They were standing there under where the bell was so I guess their pathfinding didn't realize they were not at the same level as the bell. Now that I've got rid of that small 2nd story antechamber, and just put the two staircases in a straight line no problems. I think the main thing is to give them nice simple paths, and wide paths to move through or they can get stuck.
Occasionally I do see some issue like you are talking about, but it seems to either go away, or get 'fixed' by something (such as giving them a restaurant). Try building a bar/restaurant. giving them plenty of room to move around it (or at least in front of it) and put some chairs for them (small tables are a nice touch too). I have found that, once they have a bar or restaurant and chairs to sit, they will sandbox morning and evening as if they are having breakfast and then supper.
Another thing I've noticed is that, not all "types" of NPCs seem to be able to function fully in any role. I've got some followers who are doing Provisioner role (Strong e.g.,) as well as many of the Mr. Handy robots from Grey Garden, and that seems to work fine. Cait can do farming, or "guard" (as can Preston and probably all followers. But when I've tried to assign Cait to be a bartender it didn't seem to work. Same for the Long's: assigned them to shopkeepers and they just never bothered to work their stall. Sturges works fine as a scavenger though, so maybe they would all work in that role.
Hopefully they do expand on settlements a lot in a DLC cause there are a lot of less than ideal features to it right now: ack of naming, some roles seem to "get stuck" (for example, once a Provisioner, always a Provisioner it seems) and various other small stuff.