This is a picture of a Chemical P2P Tunnel. This is hard-coded in ItemModelProvider.registerModels and reuses the texture of OSMIUM_BLOCK. The new texture was added in the 1.21 retexturing in resources/assets/appmek/textures/part/p2p_tunnel_chemical.png but not actually referenced in the code.

This is inconsistent and should be fixed in my opinion. As I'm not sure which version would be correct (keep OSMIUM_BLOCK or use the new texture) I haven't provided a PR myself, but since the fix is trivial I could do so if I know which way to go.

Found the latest dev/1.21.1, but probably also affecting dev/1.21