1. Create a setup as follows: 
   
2. Place a glass block above the green terracotta block, and hit or kill one of the villagers.
3. Try to trade with the other villager.

Result: The villager will not change the prices of their trade. This also happens when placing 3 glass panes in a line, as well as with blocks like slime, honey, leaves, snow layers above 6, and even the lectern. (I'm guessing due to the weird shape of the bounding block, its' treated as a full block). Doors and Trap doors are debatable (they have holes, but the door/trapdoor model doesn't reflect that, I think).

I know why it's happening, though - the object model is blocking the line of sight, but I would think that villagers should be able to see the villager next to them be hurt.