Fateful

Looks like MC added some things that use textures from minecraft:trims/entity/humanoid_leggings/ that have resolution restrictions. Although the previous 1.21.4 edition could load up fine in MCv1.21.3, it was getting issues with those textures' size in MCv1.21.4; causing a load failure. This version fixes the issue, and you probably wont notice a difference in quality.

For just the textures in the minecraft:trims/entity/humanoid_leggings/ directory: This version has a triple scale (256x128 → 768x384) from Faithful x64 rather than the normal quadruple scale (→ 1024x512). Other textures remain unchanged from the previous 1.21.4 edition, and this seems to allow the resource pack to load in MCv1.21.4.