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 ladders 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 ladder
View the specific dark pixel on the right bar
View the exact same section of the ladder on the direct opposite side
Note that that dark pixel is now on the opposite side even though it shouldn't be
Can confirm in 20w48a.