Since it looks like this could affect a variety of things and may explain a number of other bug reports, I'm giving this its own post to make the information I found as easily available as possible. This appears to be the result of some areas of settlements not being correctly keyworded like the main part of their settlement and the affected areas can cause some of the following effects:

• Food vendor stands, bells (to attract settlers), caravan trader outposts, and brahmin troughs (both types) don't appear if build mode is activated while standing in the area.
  
• The above objects become faded and unusable if moving into the area with the build mode already active (and changing the item selection at all). This only applies to initial construction; stored objects remain unfaded and can be placed as normal.
  
• Vendors (not just food vendors) in the area don't offer their barter dialogue, instead giving just normal settler trading or generic dialogue.
  
• Machine gun turrets placed in the area don't appear at the maximum turret level used for the rest of the settlement.

There could be other effects or similar area problems, but those are what I know so far. I also ran around my own settlements using the second effect to map them out. They all split along strictly cardinal lines (north/south/east/west), and you can imagine these as the grid dividing one piece of map from another. Here's a general description of the places for each settlement where I found an issue (some of the measurements are in standard foundation block sizes to help indicate their size):

• Abernathy Farm: 2 foundation block thickness of north corner, also everything south or east of map icon's south-east corner.
  
• Boston Airport: Very small notch along middle of south-west side.
  
• Bunker Hill: West edge, outside settlement wall.
  
• County Crossing: 1 foundation block thickness north corner, 3 blocks thickness south corner.
  
• Echo Lake Lumber: Very narrow west fringe, similar to Greentop Nursery.
  
• Egret Tours Marina: 1 foundation block thickness of east side, also north-west corner, 5 blocks wide east-west, 1 1/2 blocks north-south.
  
• Finch Farm: North side starting level with roughly 1 foundation block thickness of north overpass support pillar, also 1 foundation block thickness of south-east corner.
  
• Greentop Nursery: 1/2 foundation block thickness on west side.
  
• Kingsport Lighthouse: 1 foundation block thickness of south corner.
  
• Longfellow's Cabin: Everything outside the fog condensors except to the east; everything north of the northmost two, west of the westmost two, and south of the southmost two.
  
• Murkwater Construction Site: 1 foundation block thickness of west side, 1/2 block of north side.
  
• Nuka World Red Rocket: 1 or 2 foundation blocks thickness of east side, level with and south of the main building's brick structure.
  
• Nordhagen Beach: Almost all water on the south and west sides, at 3 or 4 foundation blocks thickness of east corner.
  
• Oberland Station: 3 foundation blocks thickness of north side.
  
• Outpost Zimonja: Extreme north corner, roughly 1 foundation block.
  
• Red Rocket Truck Stop: Roughly 2 foundation blocks thickness of north corner, 1 block of south corner.
  
• Sanctuary: Sizable chunk covering south bridge and water to the east of it until roughly the narrowest point of the water on the map.
  
• Somerville Place: Roughly 3 foundation blocks thickness along the northern side, 4 along the west, and 1 along the south.
  
• Spectacle Island: Area slightly south of mirelurk queen boat, and block area covering the grounded boat on the west shore, slightly north of it and everything south and west of it.
  
• Starlight Drive-In: 4 foundation blocks thickness of east corner.
  
• Sunshine Tidings Co-Op: South-west corner, covering SW house and silos
  
• Taffington Boathouse: Extreme west fringe, plus everything beyond the southernmost edge of the walkway around the house.
  
• Visitor's Centre: North corner of the visitor's centre, roughly beyond the northernmost fog condensers if they're installed.

I did not find affected areas in the other settlements (Coastal Cottage, Covenant, Croup Manor, Dalton Farm, Graygarden, Hangman's Alley, Jamaica Plain, Tenpines Bluff, The Castle, The Mechanist's Lair, The Slog, Vault 88, Warwick Homestead). They could have other area issues, just not this one.