For a comprehensive list of other examples of issues with 2D planes in models not being mirrored as expected, click here.
The bug
The planes composing chains do not mirror on the back as one would expect, resulting in physical/illogical weirdness.
Fixed model (Mojang has full permission to use this fix):
[media]
How to reproduce
Place a chain
View a specific section of the chain in question
View the exact same section of the chain on the direct opposite side
Note that the exact texture pattern at that position is different, even though it should be the same
Hi, I can confirm for 1.16.4 prerelease 2.