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 vines 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]
This issue did not exist up when vines were introduced, and was introduced with the fixing of MC-48512 in 14w10a.
How to reproduce
Place a vine on a sufficiently visually transparent block or hanging
View the jutting out pixels on the bottom right of the vine texture
View the exact same section of the vine on the direct opposite side
Note that the exact texture pattern at that position is different, even though it should be the same
Linked issues
Attachments
Comments 4
Can confirm in 21w05b.
Can confirm in 21w06a.

The issue hasn't been fixed completely. Up & Down faces are still not mirrored correctly.
[media][media]
Can confirm in 20w51a.