This is NOT a bug with the look of the texture, it is an error in how it is mapped

Affects:

• Piglin arms, legs, ears
• Zombified Piglin arms, loincloth, legs (compare to old Zombie Pigman texture), ears
• Possibly affects Strider legs as well

These textures have quite different shading for inner and outer parts of arms and legs and are obviously intended to be mirrored on each side.
(This was not an error in mob textures prior to 1.16, as they have their appendages mirror each other. A good example is the Pillager and a good direct comparison is the Zombie Pigman leg textures vs the Zombified Piglin leg textures)