I know this mod hasn't been officially updated for 1.21.4 yet but since it loads in game without errors, I figured I'd mention this. If the issues not related to other mods are already known and being worked on, I'm sorry for wasting your time.

I also saw in a closed issue report (#89) there was someone experiencing some of the same things I am and they posted a list of their mods. I went through the mods I had in common with them and was able to pinpoint the mod causing one of my issues and possibly the other user's only issue - Sodium.

My issue is a tad more complicated though since part of my issue still exists after removing sodium. My game shows a missing texture in the inventory, in hand, and on the ground. When placed properly the copper hopper looks like a regular hopper with a mesh in it. Without Sodium installed I get the correct placed texture but the ground, hand, and inventory textures are still missing. Here are my images showing what I'm seeing and my relevant game folders:

With Sodium
https://drive.google.com/file/d/18avT9FuA1Fmzt2f1pcXzAo7bB53j0Z88/view

Without Sodium
https://drive.google.com/file/d/1885T8dhgRPjRBkvqYwVNNG_1ZTtTxdmk/view

If you want to see all of the images in their original sizes you can view the whole Google Drive folder here:
https://drive.google.com/drive/folders/1kVwAauRJFngnqYu1mmWjD0u4WOJHPC8R

The screenshot of the placed hopper with the correct textures (without Sodium) does have an extra datapack that was removed in the later images but I left the image there since it was the only one showing it working as intended anyway. It obviously works without the external datapack too.

Last thing I'll note is that I have also disabled all resource packs as well before taking these screenshots as well. Figured I'd mention that since you can't see that in the images of my folder structures.

Functionality works fine so far. It's just the textures that I've seen so far. If there's anything else I can provide to help, please let me know.

Pretty sure this is a dup of #89, which was recently fixed. LMK if I missed something.