Any new textures added to an existing resource/behaviour pack, whether a new mobs texture, or just a retexture of a block, show as an error

Textures that have been changed in previous versions seem fine, but any new textures do not show up, even if you had changed that texture previously with no issue.

Edit: seems to particularly be bamboo planks, cobbled deepslate and tuff? Tried using bamboo planks as a safe testing block, and it showed up as an error? Will still link the texture pack. Bamboo planks should be copper grate, coarse dirt should be some white block, and dirt should be greyed out grass block. Pack icon should be pack.png, though its always been an error. 

Steps to recreate (texture pack)

1. Unzip the folder and place it in the resource packs folder within minecrafts files. Look up the location, as its different across devices
2. Load into a world and apply the texture pack.
3. Exit the game.
4. Get a 16x16 image, name it after a block (some may not work because of where they're placed in texture packs), and place it into Test Block Textures/textures/blocks. [Adding a deepslate folder within blocks, then adding a 16x16 image named "cobbled_deepslate" tends to result in an error. Tuff.png also does.]
5. (Optional), following the same format, add the directory to the texture_list.json file. The texture pack will work whether or not you do this.
6. Open the manifest.json file and change the UUID, you can just change a couple numbers. Change the name to make it more identifiable.
7. Remove the original texture pack from the world/delete it if possible. Add the new one.
8. Place any changed blocks. Some may be errors.

Will link images as this is easily recreatable.

First noticed this in 1.21.44. Will create a random texture pack that changes one block to further test it, and link it, as the resource/behaviour pack combo I've been using may be very large at this point. Will edit this further and add steps to recreate if this turns out to be consistent. This has been bothering me for a few weeks, and recently affected a development resource/behaviour pack combo that I hadn't changed the textures of, and used entity wizard to begin with. First time a mob got affected.