The zombified piglins are showing the old zombie pigman texture when any texture pack is applied, even if it does not mess with its texture. The uploaded pictures shows clearly that the texture pack is applied but it changes the look of the zombified piglin. The resource pack I used is also uploaded as a zip file, so you can check if anything affects the zombified piglin’s texture.
The problem is in the manifest.json file.
[media]min_engine_version
is on the wrong line. I've attached an updated manifest.json file that fixes the issue.