Versions:
2.2.4 for Minecraft 1.12.2
4.1.6 for Minecraft 1.16.5

Description:
"Small" shields held by the player in their off-hand (NOT blocking) are held slightly off from what is expected. This results in minor clipping. Occurs identically in both versions.

This occurs with the buckler-type shields that do not render with a ring around the player's arm like the vanilla "tower" shield. A buckler-type shield from the mod Spartan Shields is used for the screenshots.

The block animations also look off in my eyes but this may be intended.

Main hand idle:

Main hand block:

Off-hand idle:

Off-hand block: