If you put a Shield in your left hand, hold right click, and pass through a nether portal, the shield will remain active as long as you keep it in your left hand. You can continue to mine, attack, place blocks, eat, use other items.

In short, you are literally invincible if you are attacked from the front, but you will still take damage if you are attacked outside the scope of the shield.

This bug occurs since 1.9 and probably affects 1.17 snapshots