There were big changes in 1.21.2 to entities and models, many packs will need to be updated,

check if your issue is mentioned here before posting
https://github.com/Traben-0/Entity_Model_Features/blob/master/.github/1.21.2%2B%20%20changes.md

If it is mentioned there go and report to the resourcepack maker instead

Common issues are:

• broken baby model animations
• broken humanoid 'headwear' animations
• broken 2nd layers in player animation packs.
• armor textures being broken