I believe this may be an issue with the updating process, the same thing happened to me on Windows 10. I had to fully reinstall the game to get the new Zombified Piglins and Sound Effects.
I'm seeing the exact same issue, 3pix (alex) arms has the left leg's texture inverted, whereas the 4pix (steve) arms have the correct texture orientation.
I believe this may be an issue with the updating process, the same thing happened to me on Windows 10.
I had to fully reinstall the game to get the new Zombified Piglins and Sound Effects.