Items in offhand render funny when in nether portal

Whenever you are standing in a nether portal block, the item in your offhand will disappear (unless it's a shield) and the item in your main hand will look very weird, and appear to have been cut off by an invisible box. 

Sidenote: this is not caused by my resource pack, I did the same thing in the video with all resource packs disabled and the bug was still there.