I managed to figure it out, but I am very inexperienced with resourcepacks so I'm unsure, but my suspicion was that the creeperhead block is now just created directly from the entity texture, making it locked in at 64x32 resolution or it wouldn't display correctly. Any resolution over that would just make the heads display the entire entity texture uv almost.

The only way to fix it for me was to recreate the textures and models in blockbench, getting a template from a default creeper cem template and recreate the models, limited to only the short stubby creepers, and keep their texture resolution to 64x32, and not move or change the head uv from its default position. Would be happy to send you my remix of your pack to help with this if needed! But also just want to say I'm not distributing my version of this pack anywhere, but was so obsessed with your designs that I was determined to figure it out for myself!