There are some blocks that donot render correctly in head slot
/replaceitem entity @e[type=Villager,c=1] ...

• Rails (is not a full block, different block model) = displays as item.
• Hopper (the texture is not generated from the block, like other blocks such as stone or dirt) = displays as item
• any armor or weapon (villagers cant wear armor) = invisible (still succesfully runs command and says it placed the helmet but it actually didnt)
• (LOL) levers = displays as item (looks like antenna)
• same with: torches redstone torches, tripwire hooks, flowers, panes, bars, vines, ladders, tallgrass, double_plants, water lilly, cobwebs and other 'item model differs from block model' items.

Certain blocks cant be placed in the headslot despite the fact it is a valid block

• cauldrons (why can hoppers be placed but cauldrons not?) = gives error
• brewing stand (same thing) = gives error

Summary:

• Items that have a block model/texture that differs from the item model/texture will display as the item, rather than the block (2D) such as: hoppers, levers.
• Blocks in the tab Brewing donot work. (Cauldron is definitly a block... so why doesn't that one work?)

(btw do villagers never drop items? because when they have full bread slots and head slots even then they dont drop those!)