Attempting to use a resource pack to customize the shadow.png texture will cause it to break and appear in a 2x2 block formation. The attached video is me using the attached resource pack to customize the shadow texture in 1.16.5.
I have tested this with two resource packs, both making it into a square. Both end up breaking it, so this one will do just fine.
It's possible that other textures may not break, but these ones do, and it that case, that would be the only fix that needs to be applied.