The Bug:

During 1.14's Texture Update, Pigs were given a new texture with new features like eyebrows and more professionally done shading. However, the Pigscene painting, which uses a stretched version of the original Programmer Art pig, was not updated to match. 

This pig was not a part of the original painting by Kristoffer Zetterstrand, but is instead an extra addition to the piece by Mojang. As such I would expect it to reflect the current design of pigs:

As-is, it is a remnant of old, outdated Programmer Art that is no longer present in the game by default.

Steps to Reproduce:

1. Get a Pigscene painting and look at the pig head's texture
2. Spawn a pig next to it with the default resource pack, and observe how the pig's most recent texture differs greatly than what's depicted in the painting