Blocks in offhand slot render incorrectly

When blocks are placed in the offhand slot using the "/replaceitem entity" command, the block renders incorrectly. 

Steps to Reproduce:

1. Load up a creative world with cheats

2. Open the command window

3. Type in: /replaceitem entity @p slot.weapon.offhand 0 block_goes_here

4. Observe

Note:  This does not happen with items (like the Firework Rocket) or blocks that do not use their block model as their inventory texture (like the Barrier Block) 

Observed Results:

The block is rendered tiny on the left side of the screen, not connected to the offhand. 

Expected Results:

The model should be located on the offhand, same size as the main hand, essentially mirroring the main hand and Java edition.